Building an Amateur Radio Receiver

Building an Amateur Radio Receiver
Gene Dorcas, W5DOR
Dec 17, 2:23pm, 2014
Here I'm just documenting my thoughts and progress of designing a homebrew receiver.
As I proceed with this documentation it keeps looking like a book and much more than I
ever intended to write. This is not really intended to be a book for publication but
instead, a set of notes that I collect as I develop my receiver. My hope is that someone
might find these notes useful.
Today I'm adding notes on actually building a receiver. See Chapter 11 .
I had this enclosure and some knobs and think my first simple superhet receiver might look something like this. I like a big tuning dial knob
and a digital frequency display.
Introduction:
By using commercially manufactured ham gear from the time you first receive
your ham license to the present you are depriving yourself of the complete thrill
that a ham receives when operating his/her own designed and built gear. If you
have never built a ham receiver, perhaps this is the time for you to gather some
parts, heat up your soldering iron and get busy. Ham radio homebrew provides a
variety of exciting experiences. Foremost is being able to operate successfully a
ham station that has been built on your own workbench. In this book we'll look
at the receiver in its simplest form and then progress by adding features to
improve its performance.
A piece of ham gear built from a kit is not a homebrew rig by definition but if
you're hesitant about getting into the homebrewing arena you may want to start
off with a kit to get your feet wet with soldering and assembly techniques. Some
of the kits on QRPme.com or 4SQRP.com are very good. I've built the 4SQRP SS-40
receiver and the NS-40 transmitter. Both worked very well. I haven't built any
QRPme kits but they must be good because they have quite a following. If you
decide to use "manhattan" style assembly you'll want to purchase some of the
MeSquares and MePads from QRPme.com Observe the use of MeSquares and
MePads in the work of Dave, AA7EE . (Scroll down to where Dave begins to add
the circuitry. Doesn't he do beautiful work ). Several of Dave's blogs show how to
make nice enclosures from pc board material.
My lifetime experience as an electrical engineer was 99% in the digital world and
when I retired and took up my ham radio hobby again I realized there's lots of
analog circuits and designs that I'm not familiar with. This book will be full of
published circuits that are tried and true because I want to have some degree of
success as soon as possible.
In the beginning, back in the 1930's when my dad was a ham, everyone built their
own receiver. He and I built a short-wave receiver from scratch when I was a kid.
He used some of the coil winding data from his 1936 ARRL handbook. Of course
it used vacuum tubes. It had hand-wound coils, band-switching, 3 tubes and was
powered by a B+ battery and a C battery. We listened on some old crystal
headphones. I didn't know the morse code yet so I listened to foreign broadcast
stations and we used that receiver for several years. When I finally got my ham
ticket dad bought a surplus BC-342 receiver that worked quite well, or at least
from my experience it seemed to work very well. When I got my general ticket a
year later we bought a HQ-170 receiver and a Johnson Viking II transmitter. I
worked the world on 20meter phone (AM). I still, however, constantly had in the
back of my mind the desire to build equipment of my own. With the integrated
circuits and the modern day components available today one can build his/her
own receiver with performance approaching state-of-the-art commercial
equipment..
Most homebrewers building their first receiver will start with a direct-conversion
or regenerative receiver because of its simplicity. However, reception can be
improved dramatically when moving from the simple DC (Direct-Conversion)
receiver to the basic superheterodyne receiver. Some of the most noticeable
improvements are much better selectivity, single-signal reception, and
elimination of microphonics and common-mode hum. Most superhets will also
be able to include AGC (automatic gain control) circuitry and a S-Meter.
The most used piece of equipment in the ham shack is the receiver. It facilitates
communications with other stations and is also a much needed piece of test
bench equipment. The PHSNA Yahoo group is building a test receiver for use on
the bench.
After reading "Crystal Sets to SSB - K0IYE" by Frank Harris, K0IYE, ARRL
Handbook(s), EMRFD, Solid State Design for the Radio Amateur by Wes Hayward
W7ZOI and Doug DeMaw W1FB , and Doug DeMaw's books, "Design notebook"
and "QRP Notebook", I began to develop a plan to build a ham receiver. I also
communicated with Frank Harris, K0IYE, via email. His book is valuable because it
goes through the thought processes Frank went through while building his
receivers including the errors and problems his receivers had and how he
overcame them. His latest version is Rev.13. As I learn about receiver design I
believe the best starting point for anyone that wants to learn about receivers is
the Receiver Design Basics chapter in the Solid State Design for the Radio
Amateur. That publication has been out of print for many years but is still a very
good resource for the homebrewer and can usually be found for sale on
amazon.com.
Since I have a great deal of experience with integrated circuits I will use them to
the best advantage in this receiver design. I also plan to use a small
microcomputer to make the internal functions easier to use. I think the Arduino
Uno or Mega 2650 will work just fine. The Arduino programming is done in a
simplified version of "C" and since the Arduino is "open source" there is an vast
amount of support. I will have to shield these digital signals well to keep down
the noise they might generate in the more sensitive analog receiver circuits.
M0XPD shows using the Arduino for oscillators in his projects.
Be careful using digital ICs in a receiver !!! They radiate all
kinds of noise and have to be treated with special care to be
used successfully. Their output has to be filtered well to yield
a pure sine wave signal for input to a mixer or detector. You'll
have to take special precautions in shielding any circuit
carrying digital signals. We'll discuss it in detail later on.
Lately I've been reading about using DDS and was thinking I might be able to
successfully use two of the inexpensive AD9850 modules available on eBay, one
for the LO and one for the BFO. Then I found info on the web about the Silicon
Labs SI5351, a DDS unit with multiple outputs each one able to produce a
different frequency independently.
My electronic experience started at the age of 10 when my dad started teaching
me electronics in my pursuit of a ham license. I received my novice ticket in 1955.
My brother went through the same process and is now K5DOR.
I operated from my home in Ft. Worth, Texas until I became a US Navy radioman
in 1961. I had wanted the Navy to train me as an electronic technician but when
they found out I knew the Morse Code they sent me straight to radio school. I
operated US Navy communications equipment as a CW operator for years aboard
the USS Bushnell based in Key West, Florida. I held a speed-key certificate that
was required if I wanted to use a bug. I copied code for hours at a time day after
day on a mill (mechanical typewriter). One finger was used to type 30 wpm while
the other hand was used to drink my coffee and smoke my cigarette. Luckily I
stopped smoking the day I was discharged. Since I had originally wanted to be an
electronic technician I completed a two year correspondence course in basic
electronics while in the Navy. When I was discharged I immediately began to look
for an electronics technician job in the Ft. Worth-Dallas area where I was
originally from. When I applied at Texas Instruments I was rejected because they
said I had no formal training in electronics. I asked if they had a test they could
give me to show my skill and to my good fortune they did. I aced the test and
became an electronics technician in an integrated circuits development lab. TI
was very gracious allowing me to immediately register as a electrical engineering
student at the University of Texas at Arlington where the G.I. Bill paid me each
month for attending college and TI paid for my tuition and books and allowed me
to work whatever schedule fit my school work. During my career I worked for TI,
TRW and Lockheed.
I frequently thought back to the ham radio days and always wondered what it
would be like to be an active ham again. Now I'm retired and have plenty of time
to devote to the art of ham radio. I've always wanted to build my own radios and
now I can. I remember as a teenager lying across the bed for hours browsing the
Allied Electronics catalog wishing I could afford to buy parts to build my own
equipment. The other thing I wished for was a Collins 75A-4 receiver, which, back
in 1955, was the ultimate in ham receivers. I now have my own 75A-4 that has
been restored and has all three mechanical filters. It's still a very sensitive
receiver at a MDS (minimal discernible signal) of about -140dB. I've increased its
performance outboard with the addition of a DSP DNR module that reduces the
noise and a non-ringing 200Hz audio filter. It's still a very nice receiver.
My idea of a homebrew receiver includes a DDS (direct digital synthesis)for the
local oscillator (LO) for stability, the ability to have a digital readout and easier
band switching. The NE602 (or NE612) mixer will serve well as a 1st mixer and
product detector. I plan to switch between a couple of crystal filters, one for SSB
and one for CW. For AM I will probably use a different type of detector. For
extremely narrow bandwidth I'll switch in the 200Hz Hi-Per-Mite filter. DSP DNR
(Digital Signal Processing - Digital Noise reduction) will also be used to decrease
the atmospheric noise level. The receiver will be band switched and provided
with mute circuit(s) for QSK operation.
M0XPD has experimented with using two inexpensive eBay DDS modules
controlled by an Arduino as both the LO and the BFO. Since we're going to use an
Arduino as a digital controller we'll follow his work as closely.
Another aid to experimenting with the receiver circuits is to purchase a couple of
sets of K8IQY Designer Dream Boards (DD) from 4SQRP group. These are pc
boards with a mix of useful circuits laid out for receiver or transmitter designers
to use without having to lay out their own boards.
The DD board for receivers has separate circuits for LPF, BPF, two LT1253 RF
amplifiers, a 4-pole crystal filter, a Colpitts crystal oscillator, a power supply e
regulator and conditioning circuit and a couple of bipolar transistor switches.
The DD board for transmitters contains a driver and final amplifier circuits.
The DD board for receivers uses the highly successful circuits used in the high
performance SS-40 receiver.
If you're interested in building a QRP transceiver I suggest the version of the SS40TX that will output up to 9watts but can be adjusted for the QRP Full Gallon
(5watts). Be aware, however, that you will be confined to the small portion of the
band covered by the VXO of the SS-40 receiver. The solution for many has been
to connect an outboard variable oscillator for the LO of the receiver and oscillator
of the transmitter. I would suggest possibly using the N3ZI DDS 2 Super DDS
because it includes a RIT function. An amplifier must be used, however, because
the DDS doesn't output enough voltage to drive the input of the diode-ring type
mixer of the SS-40. Suggested amplifier circuits are included on the web site for
the N3ZI DDS.
Table of Contents
1. My Receiver Specifications
2. My lab and "junk box"
3. My current rig(s)
4. Building sequence
5. Receiver Design
6. Stability
7. Sensitivity
8. Selectivity
9. Testing and Measuring
10.Tuning Frequency Accuracy
11. Starting to Actually Build
12.The IF Amplifier
13.Selecting Components
14.Building My Own PC Boards
15.Noise Blanker and Noise Limiter
16.Crystal Filters
17.Using an audio filter
18.Making the Inductors
19.IF Cans
20.The BFO Circuit
21.Switching Bands and Filters
22.Reducing noise in from the DDS
23.The audio amplifier
24.Single-Sideband Operation
25.The Power Supply
26.The Enclosure
27.Painting and labeling
28.Parts Sources
29.Records Keeping
30.Bibliography
1. The Receiver Specifications
This receiver should include:
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Ham bands 160m thru 6m
Have careful gain distribution
High sensitivity along with high dynamic range
Noise blanker along with digital noise reduction
High performance IF derived, IF/RF controlled AGC
Digital logic control with a versatile microcomputer such as Arduino
Intuitive push-button controls along with large tuning knob(s)
Relay switched crystal filters
Front-end attenuators, 6dB, and 12dB (1 & 2 S-units), relay switched
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60Hz filters at power supply input and in audio section
RIT with separate knobs
Clear sharp audio with plenty of drive for a reasonably large speaker
Intuitively located front panel controls
Large single line LCD for frequency
Smaller 2 x 16 LCD for remainder of displayed info
Push-ON, Push-OFF button for display back-light
Push buttons for increasing or decreasing frequency
A number of frequency memory channels
IF shift. Move IF channel through the IF filter bandpass
Frequency stability better than +1ppm/hour
Frequency steps, 100Hz, 10Hz and 1Hz
Sensitivity 0.2uV, Selectivity 2.1KHz SSB, 0.5KHz CW, 0.2KHz CW
Audio output >2w
Front panel includes a large tuning dial, A & B VFO switch(s), RF Gain, IF
Gain, AF Gain, Large LCD freq. display, Smaller 2 x 16 LCD for additional
infomration, Mode selection, BFO adjust, Calibrator ON/OFF, Noise Limiter,
Noise Blanker ON, AGC Threshold, AGC ON/OFF, IF Shift, DDS function, freq
step UP and DOWN pushbuttons, and power ON/OFF.
Of course we want the three S's, sensitivity, selectivity and stability. It also has to
provide a certain amount of gain - 90 to 110dB. Additionally, we want an
accurate digital frequency readout, a large tuning knob with very smooth action (I
just like turning that big 'ol knob.), band switching, the ability to handle very
strong signals while at the same time being sensitive to very weak signals
(dynamic range), ability to select USB, LSB, CW, AM and digital modes, automatic
gain control (AGC), Rf Gain, Antenna Tuning, Noise Blanker, Noise Limiter, IF
Gain, IF shift, Audio Gain, Band-Pass tuning (BFO control)and a S-meter. Another
modern day feature is RIT or Clarifier which allows a CW (or other
modes)operator to successfully receive on one frequency while transmitting on
another but nearby frequency. If you are a CW DXer this is almost a requirement.
I like to hangout on 10m CW in Spring and Fall and the Clarifier in my FT-450D is
used more often than not. When that DX station sends "CQ de W5XXX UP " He
means for you to call him up from his frequency 1 or 2 KHz. If there's a really big
pile-up you may have to call him even higher up. The DDS I am using has the RIT
feature built into the software. It includes VFO-A, VFO-B and RIT functions. (N3ZI
DDS 2)
The goal of this project is to build a receiver that performs well. It won't be better
than those that are commercially available but hopefully close. We will, of course,
use parts from out junk box if possible but don't plan to sacrifice performance
because of cost. but be aware that I can say that because I have a very large "junk
box".
To have the best sensitivity we need to look at the front-end closely. We also
have to make sure other sections of the receiver don't add noise covering up the
weak signal we're trying to hear. From the antenna the signal will go through a
band-pass-filter (BPF) and into the 1st mixer except on the higher bands, 20m
through 10m, where a preamp can also be useful. On the lower bands a preamp
doesn't buy us anything because the atmospheric noise is too high and covers up
those weak signals. We will use double-tuned circuits (DTC) for the input filters
each designed specifically for their band. Ideally the BPF will have only a couple
of dB attenuation, will be flat across the band and will attenuate any out-of-band
signal significantly. We'll have to design the BPF for each band for the best
performance and test it with a scalar network analyzer such as built by the PHSNA
yahoo group (PHSNA = Poor Ham's Scalar Network Analyzer). I do, however, own
a Rigol DSA815-TG spectrum analyzer that I will use to test and adjust the filters
for best performance.
The NE602 mixer IC we plan to use may not be able to handle an extremely
strong signal, therefore, sometime in the future we may decide to change the 1st
mixer to a ring diode type that can best handle the large signal. If we do that we
will have to add some gain to make up for the gain in the original NE602. On the
higher bands that additional gain can be found in the antenna preamp so a
solution may be to use a preamp for all the bands feeding the 1st mixer as a ring
diode type. A ring-diode mixer with built-in impedance matching transformers can
be had from Mini-Circuits. The ADE-1ASK is the surface mount mixer I have in my
junk box and is good from 500 KHz to 500MHz. it is available at a very reasonable
price at KitsandParts.
Building it this way will allow us to layout a standard front-end pcb that will
contain the BPF and preamp. These can be built for each band and then selected
by a multiplexer that is in turn controlled by a rotary band switch or by a microcomputer (uC) or both. The preamp will probably be designed around a
grounded-gate J310 FET because of its very low noise figure. If you check the
JFET transistor data in the ARRL handbook you will see that the J310 is by far the
quietest. Each front-end module would look something like this from VE7PBO,
Designed by W1FB and found on qrp.pops.net: (I just found out that the
QRP.pops.net web has been taken down permanantly. ) It's designed for a 50ohm
output so we will have to adjust the number of secondary turns on the output
transformer.
Of course the frequency dependant components will be different for each
module. The whole front-end section would also need to be in its own enclosure
to eliminate the sensitive front-end from picking up stray signals from within the
receiver or from the outside world. This is especially true if you have digital signals
in the same receiver.
This receiver should include:
























Ham bands 160m thru 6m
Have careful gain distribution
High sensitivity along with high dynamic range
Noise blanker along with digital noise reduction
High performance IF derived, IF/RF controlled AGC
Digital logic control with a versatile microcomputer such as Arduino
Intuitive push-button controls along with large tuning knob(s)
Relay switched crystal filters
Front-end attenuators, 6dB, and 12dB (1 & 2 S-units), relay switched
60Hz filters at power supply input and in audio section
RIT with separate knobs
Clear sharp audio with plenty of drive for a reasonably large speaker
Intuitively located front panel controls
Large single line LCD for frequency
Smaller 2 x 16 LCD for remainder of displayed info
Push-ON, Push-OFF button for display back-light
Push buttons for increasing or decreasing frequency
A number of frequency memory channels
IF shift. Move IF channel through the IF filter bandpass
Frequency stability better than +1ppm/hour
Frequency steps, 100Hz, 10Hz and 1Hz
Sensitivity 0.2uV, Selectivity 2.1KHz SSB, 0.5KHz CW, 0.2KHz CW
Audio output >2w
Front panel includes a large tuning dial, A & B VFO switch(s), RF Gain, IF
Gain, AF Gain, Large LCD freq. display, Smaller 2 x 16 LCD for additional
infomration, Mode selection, BFO adjust, Calibrator ON/OFF, Noise Limiter,
Noise Blanker ON, AGC Threshold, AGC ON/OFF, IF Shift, DDS function, freq
step UP and DOWN pushbuttons, and power ON/OFF.
2. My Lab and "Junk Box"
My work bench and adjoining "junk box" have evolved. Seems like I've always had
a workbench at home but it's now out of space. I need to add a couple of more
tables along with shelves on the back of them to hold my parts bins. I have several
collections of parts that require sorting. I use small labeled envelopes for small
parts such as ceramic capacitors, NPO ceramic capacitors, molded RF chokes,
silver-mica capacitors, . .etc... Here's an example - my collection of NPO ceramic
capacitors.
I also have ways to test almost all of my parts. Of course I have a digital multimeter that can be used to test resistors. Test capacitors or inductors I highly
recommend the AADE LC meter. I also have a PEAK DCA75 which will test almost
all of the discrete semiconductor parts. Digital and analog integrated circuits must
be tested in a breadboard environment with a seperate setup for each IC.
I have my junk box divided into several catagories:
Enclosures
Passive components
Analog Integrated Circuits
Digital Integrated Circuits
Knobs and tuning apparatus
Glues and Lubricants
Connectors
PC Board Materials
There are several places I purchase parts such as:
Mouser Electronics
MPJA
BG Micro
Digi-key
My lab is in a spare bedroom. When we moved here 7 years ago we were
downsizing but still got a 3 bedroom, 2 bath home. We had to have a bedroom to
turn into a lab and an extra bedroom and bath for guests. The bedroom is my
shack and lab. It has a big closet with double-doors where I keep most of my
parts.
I have the rig on my desk on the side opposite from the workbench and it's full
also. I just have one antenna since we moved in here but we have lots of very tall
trees so I plan to erect a double Zepp for 160m and up and maybe just another
long wire for a separate receiving antenna for the work bench. Our HOA is very
understanding about ham antennas and there's several of us here in this
residential area. Even some with large towers and multiple yagi's.
My test equipment includes, as you can see above, a stereo microscope, soldering
equipment, Tektronix 465B scope, HP8640B signal generator, Rigol DSA815-TG
Spectrum Analyzer, B&H freq counter, B&H DVM, Simpson 260 VOM, Velleman
pocket DVM, 2 HP lab supplies, and HP 3400 RMS voltmeter, Return-loss bridge,
Semiconductor device tester, AADE LC meter, Step- attenuator, Noise generator,
Elecraft XG2 signal generator, PHSNA scalar network analyzer, AD8703 power
meter, and a collection of test jigs .
Notice the three spools of enameled wire on top of the HP 8640B? Well, it's the
kind of magnet wire whose insulation melts with the soldering iron heat. Makes
soldering in my toroid inductors so much easier.
There much more taking up room on the tables in the garage such as my shear for
cutting pc boards and other standard tools such as drill press, table saw, sanders.
Another table in the garage holds the pc board etching equipment. I locate that
near the door so when I etch I can blow the fumes out the door with a fan to keep
them from rusting my tools in the garage. I learned that the hard way.
It's hard to make out in this picture but my "muppet" pcb equipment is piled up
on this end of the 1st table. I have it positioned near the garage door for a
reason. When I etch, the acid fumes fill the air and rust any exposed steel tools or
other apparatus nearby. So, when etching I place a fan behind the etch tank,
open the garage door and blow those nasty fumes out into the driveway area.
Another large and important section of my lab is the "junk box". As all old-timers
know the proverbial junk box is a very valuable piece of the lab. I have too many
parts collected over the years so I have to sort and label like shown below.
3. My Current Rig(s)
Below is a picture of my current commercial rig. It's a Yaesu FT-450D and it does
about everything you'd want for a very reasonable price. The 1st October I had it,
I worked 140 countries on 10m CW.
I also have my QRP rig consisting of the 4SQRP receiver and transmitter - SS-40
and NS-40. The SS-40 is very sensitive and the NS-40 puts out a solid 5 watts.
It's now Summer and 10m (my favorite band) is dead most of the time. Sometime
I'll write about my exploits on 10m CW. The FT-450D works quite well on the
other bands also. I only have a 40-6m trap vertical from Diamond. I learned the
you don't want to have it too high. A study I read in QEX showed that verticals
with artificial counter poise, the higher you raise it the higher the angle of
radiation. This is not true of verticals that don't have the built in ground radials. I
have it at about 28' and it works extremely well. I worked well at 20' when I first
erected it but I couldn't get out to the West because the house was in the way. I
could hear Japanese stations sometimes but could never work them. Raising the
antenna 8' fixed that.
The FT-450D has a built in automatic antenna tuner but the external MFJ tuner
works better. The MFJ tuner will tune about any kind of antenna. I joke that it can
tune up a coat hanger on the end of a broom stick.
I also have an old 75A-4 Collins receiver in good working condition. It's the
receiver I dreamed about when i was 1st a ham at the age of 14 back in the mid50's. It's rated at a sensitivity of about -140dBm. I have an external enclosure that
I connect to the 75A-4 audio output that adds DSP-DNR and a 200Hz audio filter.
4. Receiver Building Sequence
The building sequence numbered below is probably correct but first I will conduct
several experiments to build up to this starting point.
I want to start with some simple receiver circuits to get comfortable with
designing various sections of a receiver. I'll start with a simple DC (direct
conversion) receiver in order to experiment with the NE602 (NE612 is the same
IC) mixer. I will refer to the DC receiver in chapter one of EMRFD but will make a
few simple changes to improve it and to use it as a starting point for explaining
how a receiver works.
I'll also build it several times using different construction techniques. The mixer,
the DTC (double tuned circuit) at the antenna input and the local oscillator (LO)
will be constructed several different ways. There are a number of ways to get to
the same point and they all involve the use of pc boards except for one.
Ugly Construction: This is not a name I've given the process but instead that's
really the name used by experienced electronic circuit builders. It is characterised
by the arrangment of parts to get them soldered together in the best way
possible according to the schematic. It looks like your soldering the components
together in seemingly haphazard arrangement. Something must be used to
solder components to, keeping them elevated off the pc board or aluminum
chassis. There are several types of standoffs that serve as tie points or if you use
a solid copper pc board material for the chassis I've seen builders use very high
value resistors as standoffs. Here is an example of "Ugly Construction" using pc
board material as the chassis. Here note the use of 10Mohm resistors as tie
points. If integrated circuits are used they are sometimes glued upside down to
the chassis and components and wires just soldered to the IC leads. Because of
the upside down IC looking like a bug (the insect) it is sometimes called ugly bug
construction or something similar. If you go this route make sure you use plenty
of solder because these solder joints have to hold the circuit together physically
as well as electrically. This circuit shown is an audio oscillator that I use to test
audio circuits. NOTE: If you don't have an audio oscillator for testing your audio
amps you can usually just touch the audio amplifier input with your finger and the
60Hz signal picked up by your body will be injected into the amp producing a 60Hz
hum from the output.
Now for some RF circuits:
A Elecraft XG2 signal generator is available in my lab and is designed to be an aide
to designing receivers. It is crystal controlled and outputs single frequency signals
- on 80m, 40m and 20m. It has two output levels 50uV and 1 uV. I will input the
50uV and then when the circuit is working I'll input the 1uV signal.
The circuit will be the NE602 mixer that is used in so many simple receivers. It has
some drawbacks but is mostly a great mixer for the homebrewer. About the only
fault I hear about is it's inability to handle very large signals. In other words, it
doesn't have a very good dynamic range.
I'll couple the input signal through a ceramic capacitor to pin 1 or pin 2 NE602
input. Either pin will work and the other input pin I will connect to ground
through a ceramic capacitor. The differential inputs of the NE602 are internally
biased so we'll always connect to the input(s) through a coupling capacitor so as
not to mess up the internal DC biasing. As far as the RF signal is concerned the
unused input is connected to ground because the capacitor is chosen to be a very
low reactance to the RF signal.
And the same situation with the complementary output pins, pins 4 and 5. We
will usually connect the outputs through coupling capacitor(s). Pins 3 and 8 are
the power supply pins - pins 3 is ground and is connected directly to a closeby
ground point. Pin 8 is the positive supply pin and will be supplied with 6 to 8 VDC.
I will probably use a 12VDC battery. The receiver will have a 9VDC regulator.
Power supply decoupling is very important in a DC receiver so we'll connect the
9VDC through a RF choke and then a current limiting resistor to a 6.2VDC zener
diode in parallel with a 0.1/0.01 decoupling capacitor to ground.
Why 0.1 and 0.01 uf? - Well, the inductance of the 0.1 starts to show up at 20m
and above. The combo will provide good decoupling from 160m through 10m.
That leaves pins 6 and 7 which are the local oscillator pins. The NE602 is
designed to have it's own local oscillator or can receive an external LO signal. For
my experimenting here I'll leave pin 7 open and inject a LO signal for testing from
my HP 8640B signal generator through a capacitor to pin 6.
The output of the mixer is the product of the two signals. The 1uV input signal on
pin 1 and the LO signal on pin 6. The NE602 doesn't require a very large LO signal
so we'll inject a LO signal of only a few hundred millivolts.
With the input signal and the LO signal applied we'll look for the output signal on
pin 5 or pin 4. Since this is a DC receiver test we'll want the difference signal to be
in the audio range.
To complete the simple DC receiver we need an audio amplifier with lots of gain
and enough power to drive our headphones or small speaker. To make matter
simple I'll use the Hi-Per-Mite filter from 4SQRP group. It's a high-gain audio
amplifier configured as an active filter centered at 700Hz with a bandwidth of
200Hz followed by a LM386 audio power amplifier. This module provides for an
external audio gain control. Well use this as a complete audio section for our DC
receiver with the added benefit of the 200Hz filter that will render our little
receiver as a nice portable CW receiver for the 40m band.
When that's all working pretty good I'll replace the HP8640B with a variable LO
module and add a band-pass filter (BPF) to the antenna circuit to complete the
receiver. The 40m BPF will be wide enough to pass through any 40m CW signal
from the antenna while attenuating any unwanted signals above or below the
40m CW band. The completed 40m CW receiver circuit will look like this:
Many suggest starting at the back end and build toward the front-end. I will
change that sequence for the reasons that follow. The best idea is to build and
test each section of the receiver separately. Building a whole receiver and then
turning it on and expecting it to work is not realistic.
Some folks say to start with the power supply but I'm going to start by using a
battery for power so I know my power supply is not introducing much noise into
the system. I'm build an AC power supply last and with an optimum performing
receiver will then know if the power supply adds any noise.
I wanted to do a few experiments with the DDS before starting. I have built the
N3ZI DDS to use as the 1st local oscillator (LO). Here's the DDS with the shaft
encoder, control switches and LCD display attached and working.
I have a DC receiver I built successfully with the QRP-tech group. I'll take that
receiver and disconnect the LO and use the DDS in its place. I have the old DC
receiver on the bench with antenna, power supply and headphones connected
and it's still working fine. Copying CW signals on 40m just fine.
Here's the sequence I have chosen for the actual build.. As I go along I may find
reasons to change it.
a. Audio power amp (able to drive speakers to a reasonable level)
b. Audio preamp sensitive enough for a DC receiver. This preamp may be able to
be eliminated in the final version. May use my TS922/LM384 design.
c. Include a volume control between the preamp and power amp .
d. Add the 200Hz audio filter like the 4SQRP Group's Hi-Per-Mite and test its
performance with an audio signal generator and then with a simple DC receive
front-end. Notice that the Hi-Per-Mite uses a LM386 power amp. I personally
prefer an audio amp with a little more power so I might use a TDA1015, a 4watt
amp that includes an independent preamp. I can insert a volume control, a
200hz filter, an audio muting FET between the preamp and the power amp.
e. I'll test this audio section with a DC receiver front-end which should actually
make a very usable portable unit out of a DC (direct-conversion) receiver.
f. Build a single 20m band front-end to test my circuit design. I'll add the DDS LO
and a BPF to a NE602 to make the front-end of a DC receiver and test it with the
audio section already tested.
g. Now the bugs are worked out of the front-end design and the LO. So it's time
to remove the NE602 mixer and install the ADE-1SDK diode ring mixer. The gain in
the front-end preamp should replace the lost gain in the mixer. We should be able
to connect this modified front-end again to the tested audio section to yield an
improved 20m DC receiver. It may not sound much different but it should be able
to handle those really strong signals that overloaded the NE602.
h. The next step is a big one because now we convert this nicely performing DC
receiver into a superhet. The DDS will have to provide a different LO frequency
which will not be a problem. We'll add a post-mixer amp, and skip the next
module which would be the crystal filter(s), then a post filter amp into a product
detector. Again we can use the NE602 as the detector that will drive the audio
preamp. I plan on using a couple of LT1253 video amps in the IF at first.
i. This configuration will consist of the 20m front end with the ring-diode mixer
into a couple of LT1253 IF amps and then into a NE602 product detector. This
should now be a complete superhet receiver even though we haven't added the
crystal filters yet. The reason I haven't is because this is not the final IF amp I
intend to end up with. The final version will have one LT1253 amp post-mixer
driving a crystal filter and a HYCAS IF module that will give us 3 stages of IF
amplification, AGC, S-meter, IF gain control ...and .etc... I haven't yet considered a
noise limiter or a noise blanker. I'll have to do some research on those types of
circuits.
J. After that is all working correctly we can then build the front-end modules for
the remaining bands along with the band-switching circuits.
K. An AC line powered power supply will come last. I'll use batteries up to this
point to make sure I'm not introducing noise from a power supply. I just have in
mind a LM317 with a common-mode AC line filter in the line and a low-pass filter
and common-mode filter in the output. I'll add lots of bypass capacitors and the
protection diodes for the LM317 (see the data sheet). Of course it will be
shielded and grounded with my heavy duty grounding system here in the shack.
The cable to the receiver will be shielded to prevent outside signals from being
picked up by that line such as RF from my wireless router and Rf noise from
fluorescent lighting.
L . Putting the whole thing in an enclosure will be one of the last things to do.
Planning the front panel layout a little ahead of time will pay off when it's time to
fit it all in a box. I have four Junker HW-32/16 Heathkit enclosures to use. I can
use one of them or I have an old 75A-3 enclosure that is essentially a 19" rack
enclosure. I can easily purchase a new 19" front panel for it. My present thinking
is, however, that the HW-32 enclosure will be more suited because it's much
lighter weight. It has a chassis bolted to the front panel that will serve well to
mount the various modules on the top and bottom. I'll take some heavy duty
cleaning tools to it to make it like new and may even use the old front panel as a
stencil to mark and cut out a completely new front panel out of thick aluminum.
I'll cut a rectangular hole for the LCD display bezel and make room for a very
large tuning knob with a spinner.
Well, this is the plan. Sometimes things don't always go as planned. I'm sure
things won't always work first time and we'll go through the process of finding my
mistakes and fixing them. One of the things I promised myself I would do in this
little ebook is to document these thing that don't work and the process of finding
out why and then fixing them. That's one thing that I really like about W0IYE,
Frank Harris' book, Crystal Sets to SSB. He documents the problems he's had
along the way, how he fixed them, and that gives the reader that experience
without having to actually go through it.
For instance, he mentions several times about the noise that having digital
circuits inside the receiver can cause. We live in a digital world nowadays and I
plan to investigate ways to overcome the problems associated with mixing digital
circuits and sensitive analog circuits. In fact one of the first things I'll be doing is
making a simple receiver front-end out of a NE602 mixer and a N3ZI DDS-2 VFO.
This will end up being a complete receiver right from the start. We'll connect the
antenna to a crude BPF (band-pass filter) that directly feeds the NE602 mixer. The
LO (local oscillator) will be the DDS unit connected directly to pin 6 of the NE602.
It's convenient that the output level of the DDS is just about perfect for the LO
input to the NE602. The N3ZI DDS already has a coupling capacitor at its output.
We'll attach a simple LM386 audio amplifier to the NE602 output pin4 or 5 and
that will complete out first receiver. From experience I expect the receiver to
work fine as far as sensitivity goes but will fall way short on selectivity. The other
one of the three "S's" will be taken care of because of the crystal stability of the
DDS. I think, because I already have one built, I will add the 4SQRP Hi-per-Mite
200Hz filter to give us the second "S", selectivity. If you wonder about the
effectiveness of the 4SQRP filter go to Dave, AA7EE page where he added the HiPer-Mite filter to his little DC receiver. After you scroll down his page, marveling
at his really neat construction along the way, find the videos at the bottom that
demonstrate how well the little filter works.
Another possibility is the use of a phasing DC receiver such as the R2. Refer to
your ARRL handbook for a description. You should be able to find "R2" in the
index.
The main difference is that we use 2 mixers, one with the LO signal and the other
with the LO signal changed by 90 degrees. These signals are then summed in such
a way as to eliminate one of the sidebands yielding true single signal reception.
Then adding active filtering in the audio section wil yield a great little CW/SSB
receiver.
5. Receive Design
In this chapter I'll attempt to explain my idea of a good homebrew receiver. I
won't attempt to use all the SDR (software defined radio) techniques used in
modern day transceivers. I will, however, use all the modern day techniques that
I can such as DDS (direct digital synthesis) and DSP-DNR (digital signal processing
- digital noise reduction). I will also try to use and explain about a direct
conversion (DC) receiver with single signal reception by means of a phasing
technique. In the audio section after the phasing out of one side-band we'll use
an active filter to obtain superior CW selectivity resulting in a very useful, highperformance CW receiver. I will then use that receiver along with a 5W
transmitter to yield a high-performance CW single-band transceiver.
I have some ideas about an unconventional method for boosting the transmitter
output to 5W from the 1.5w level of most transceivers in the NE4040 class.
However, please be aware that on the receiving end there's not much difference
between what a 5W signal will sound like and a 1.5W signal.
A couple of years ago I was in QSO with a CW mobile station and gave him a RST
report indicating a very poor tone. He came back with a very nice tone and
explained that he had by-passed his final amp and gone from a 25W output to a
1W output. His tone was very nice and I couldn't tell the difference in signal
strength of the 25W signal and the 1W signal.
Now back to the superhet receiver design. . . . .
This receiver will follow the general configuration that most think of when they
hear "superheterodyne". It will have a "front-end" that selects the incoming
signal from the antenna and feed that signal to the 1st mixer. The 1st mixer will
also receive a local oscillator (LO) signal that when mixed with the incoming signal
in the 1st mixer will yield an signal at the IF (intermediate frequency). We will
filter that IF signal with an IF filter to reject all nearby signals and pass the desired
signal on to the product detector. Out of the product detector we'll get the audio
signal that represents the signal that the transmitter intended for us to hear.
Here's a simplified diagram of a superheterodyne receiver.
There are a multitude of approaches to the design of each of these basic sections.
Additional modules might involve adding a preamplifier in front of the mixer.
Another approach to preamplification of the antenna signal is to use an active
antenna. A preamplifier mounted up in the antenna eliminates loss of signal as it
travels down the antenna lead to the receiver. An example of a good active
antenna is from Clifton Labs.
The first section is the 1st mixer or just the "mixer". Sometimes we might design
a double-conversion superhet in which case would have 2 mixers. In the case of a
double-conversion circuit it's common to have the 1st IF frequency to be very high
followed by the 2nd mixer and a much lower frequency IF. Mixers come in many
varieties. The one that's probably the most widely used is the NE602. It's easy to
use but may not always be the best choice. The NE602 is notorious for not being
able to handle a wide range of signal strengths. You may be using a NE602 mixer
that works quite well for the normal signal strengths but when a very strong
signal comes through it overloads and all kinds of signals might come out of it. A
better choice is the ring-diode mixer with an input and output transformer. The
downside of the diode-ring is that it has no gain. The loss of gain must be made
up somehow, usually with a preamp or more IF amplification. Diode-ring mixers
with built-in transformers by Mini-circuits can be purchased from KitsandParts at
a very reasonable price. An good example is the SS-40 receiver by 4SQRP. Notice
that it includes a preamplifier. The result is a receiver with an exceptionally good
sensitivity - as good as or better than most modern day, commercially available
transceivers. See the comparisons that Elecraft has put together here keeping in
mind that the MDS sensitivity of the SS-40 is -132dBm. (My Collins 75A-4 has a
spec'd MDS of-140dBm which is exceptional.) MDS = Minimum Discernible Signal.
Be aware that the NE602 is no longer manufactured so purchase the NE612 which
is identical.
After the 1st mixer comes the IF (intermediate frequency) amplifier(s). Remember
we want to spread out the overall amplification of the receiver over a number of
sections to prevent chances of unwanted oscillations. One of the first things I
learned from reading W1FB's books is that you need to spread the overall gain of
the receiver over a number of stages rather than having, for instance, a extremely
high-gain IF amplifier. Extremely high-gain amps tend to be unstable.
I will start out with half of a LT1253 integrated circuit (IC) as the post-mixer
amplifier. The LT1253 is a dual video amplifier whose gain is flat out to 30MHz.
There is a single amp version and a quad amp version of this IC. Because typical
op amps have a very high input impedance the LT1253 will not load down the
mixer. Op amps also typically have a very low output impedance which means it
can drive most anything you need it to.
In our prototype we'll keep things simple at first and just have a couple of stages
of IF amplification with two stages of the LT1253. We'll add an IF filter later. The
intention is to keep things as simple as possible until we get a working receiver
and then add enhancements later. Using the two Lt1253 amps may be just right
for a wide bandpass suitable for AM reception.
The next stage will be the detector. For SSB and CW we will use a product
detector but for our simple 1st version we'll just use a simple diode detector
suitable for AM detection. This will eliminate the need for a BFO that will be
explained later. A diode detector is as simple as they come. The waveform coming
from the IF is detected, that is, the bottom half of the waveform until all we have
is a signal varying at the rate of the original AM waveform is eliminated. Then
with some RC (resistor capacitor) filtering we'll smooth the modulation rate. The
signal which will be in the audible range will be amplified by the audio preamp
then further amplified by an audio power amp to drive a speaker or headphones.
Later we'll discuss other types of detectors such as product detectors or the
Tayloe detector. The most commonly used these days is the product detector
which is used for SSB and CW reception. A produce detector is a mixer where the
output is the product of the two input signals.
6. Stability
In a nutshell we must have components in our LO (local oscillator)and BFO (beatfrequency oscillator) that don't drift with temperature changes and the voltage
from our power supply must remain constant without ripple, noise or changes
with temperature. We will be using a DDS for the LO which relies on a crystal
oscillator for its time base and is, therefore, stable like a crystal oscillator. Another
approach is to use a VXO (variable crystal oscillator) such as used in the 4SQRP SS40 receiver but a VXO doesn't cover a very wide tuning range. If we start with a
VXO we can install a DDS later if we want. Be aware that the required signal level
for the LO input to a NE602 is quite different than for a diode ring mixer. A few
hundred millivolts is all that is required for the NE602 while 3 to 5 volts may be
required for the diode ring mixer.
Components also have to be physically stable usually to be frequency stable. In
your local oscillator (LO) and beat-frequency oscillator (BFO) your components
need to be soldered in very solid. Coils and capacitors need to be in solid so they
can't move or vibrate because of outside influences. They also have to be stable
over a range of temperatures.
What is you build a receiver and the signal drifts. You know your LO must be
drifting as it warms up but which component is the culprit? I saw this trick on a
youtube video where a guy blew through a soda straw aimed at each possible
component until he found one where his hot breath on a particular component
made the frequency change drastically. He then knew that was the capacitor that
was drifting with temperature. toroid
I use toroids to make inductors and have a variety of flat fiber or nylon washers
that I use with nylon screws to secure the toroid to the chassis or pc board. After
I confirm that the toroid has the correct number of windings I remove it from the
receiver and coat the windings with Q dope. Q dope is polystyrene dissolved in a
solvent and can be purchased for a high price. The way to do it is to make your
own. Styrofoam peanuts are made of polystyrene. Just dissolve white peanuts in
a solution of toluene or methyl ethyl keystone and you have the same thing.
7. Sensitivity
Sensitivity for the most part is built into the front end of the receiver but care
must be taken not to introduce noise in any stage that might covers up those
weak signals. i refer to the antenna input, band-pass filter and 1st mixer as the
"front end".
Overall gain of the receiver must be considered. Typically, an overall gain of 90 to
125 db is found in most commercially built receivers.
We must realize that there is a limit to how sensitive we need to make out
receiver because of the level of atmospheric noise and local manmade noise will
over shadow very low sensitivity. to test the receiver you can disconnect the
antenna and replace it with a 50ohm resistor and then listen to the noise from the
headphones. Then remove the resistor and reconnect the antenna and you
should hear a pronounced increase in noise. What that tells me is that to make
the receiver any more sensitive wouldn't produce any better results.
I plan to use J310 FET preamp on the input pcb for each band. It'll be something
like this one from the SS-40 receiver. The basic preamp here comes from EMRFD.
I'll start out, however, with a simple BPF (band-pass filter) into a NE602 (mixer).
To improve sensitivity we'll also add a DSP DNR (digital signal processing, digital
noise reduction) module to the audio section from BHi. This will have a rotary
switch to select the level of noise reduction and an ON/OFF switch. This unit will
have the following features:









Fully adaptive noise cancelling
Minimum distortion to audio signal
8 user selectable noise cancelling levels 9 - 35dB
Noise cancellation can be preset on PCB or remotely changed
Wide audio bandwidth for natural sound
Input and output level adjustment
Input overload indication LED
5 - 15VDC operation
Remote noise cancellation on/off
To measure sensitivity we are looking for the specification called MDS (minimum discernible
signal). MDS is a signal just barely above the noise level of the receiver. In other words, its the
weakest signal that can be heard.
Chart showing dBm-power-microvolts for a 50 ohm receiver input
dBm power microvolts (rms)
-134.9 3.2E-17 0.040
-133.0 5.0E-17 0.050
-131.4 7.2E-17 0.060
-130.1 9.8E-17 0.070
-128.9 1.3E-16 0.080
-127.9 1.6E-16 0.090
-126.2 2.4E-16 0.110
-125.4 2.9E-16 0.120
-124.1 3.9E-16 0.140
-122.9 5.1E-16 0.160
-121.9 6.5E-16 0.180
-121.0 8.0E-16 0.200
-120.1 9.7E-16 0.220
-119.0 1.3E-15 0.250
-118.0 1.6E-15 0.280
-116.9 2.0E-15 0.320
-115.9 2.6E-15 0.360
-114.9 3.2E-15 0.400
-114.1 3.9E-15 0.440
-113.0 5.0E-15 0.500
-112.0 6.3E-15 0.560
-111.0 7.9E-15 0.630
-110.0 1.0E-14 0.710
-109.0 1.2E-14 0.790
-108.0 1.6E-14 0.890
-107.0 2.0E-14 1.000
To test for MDS we will use an oscillator of a given output level in using a step generator
decrease the signal until we find that minimum signal we can hear.
Another way is to use my HP 8640B signal generator which can be adjusted to a predetermined
signal level down to -130dBm.
8. Selectivity
Selectivity is the ability to limit what you hear to a particular frequency or range
of frequencies, and not hear a load adjacent signals a little off frequency.
With a superhet, is accomplished by the use of an IF crystal filter of the required
bandwidth normally you would have a different filter for different modes such as
AM, SSB, and CW.
We also must consider selectivity in the front-end by means of a BPF and possibly
some degree of audio filtering.
Selectivity is normally accomplished in the IF with filters and is the ability to select
the desired signal between two or more signals that are very close together in the
IF bandwidth. I plan to have at least two IF filters, one for SSB and one for CW.
In another section further along in this book we talk about crystal filters. Here's
how I will select which crystal filter I need for each mode with relays. Again this is
from ARRL handbook 2005.
9. Testing and Measurements
In our lab here were have a HP 8640B signal generator, a frequency counter, a
couple of DVMs, a HP 3400A RMS voltmeter, a spectrum analyzer and a few
other common pieces of equipment.
First we need to be able to measure the amplitude of signals at various points in
the receiver circuit as we go along. For 40m signals we're ok because the
HP3400A is flat out to 10MHz. It measures RMS so if we need peak voltage we
just simply multiply by 1.404
So how are we to measure frequencies higher than 10MHZ. Well, I'm building an
RF probe and we'll plot its response along with the HP 3400A to judge it's
accuracy and then, for now, we'll assume the RF probe's will be fine to use up to,
at least, the top of the HF spectrum but will probably be good to much higher
frequencies.
I think I like the looks of the KK5NA Accuprobe but I want it to be on a narrower
pcb so I can mount it in a small tubular enclosure with a nice long needle point
probe. The opamp is in a SMD package and I'll use SMD components to get the
size down to what I want.
Here's KK5NA's accuprobe. I understand it's a fine instrument and if you want one
go to his web at http://www.kk5na.com/kk5na_files/AccupManual.htm . You can
find the schematic at the bottom of that web page.
So when I finish this RF probe I will test it and assume it will read accurately out to
the upper HF region. I'll have the HP3400A to 10MHz and the probe for higher
frequencies. I am planning a 11.0592MHz IF and the 3400A will measure that
with only a small fraction of error.
One procedure I'll do is to test the output of my HP 8640B signal generator and
make sure it is flat over the frequency range I intend to use it and outputs the
signal level selected. I can then test the meters assuming the output of the signal
is constant over frequency.
Now, to get started I need to test my test equipment and make sure the different
frequency measuring units read the same frequency. Then, I will use them to
measure the amplitude of the RF signal over the it's frequency range and make
sure that is reasonably accurate.
I have the HP 8640B and the Elecraft XG2 that will give me a set amplitude and a
set frequency. That will be the first test and the easiest to do.
I hope I can trust my HP 8640B to not only be accurate in frequency but also in
amplitude. It has an amplitude dial just like a frequency dial where you can just
dial in the amplitude as well as the frequency. You can also, of course, choose
what kind of waveform you want. We'll want sine wave 99% of the time.
1st test: Measure the amplitude and frequency of the Elecraft XG2 at 7.0xx MHz
at 1uV and 50uV and plot it on a Freq vs. Amplitude graph using the HP 3400A
since the 3400A is flat to 10MHz. Then I'll take several readings of the HP 8640B
from 1 to 10MHz set for the same amplitude.
Then we'll look at those signals with the spectrum analyzer and compare
readings.
I'll also plot the 5VDC voltage standard. It should read 5.0000VDC on the nose.
Another piece of simple test equipment is a step attenuator. Here's one from an
ARRL handbook. Mine, however, will have three 20dB steps. For conviencence it
will have a BNC connector on each end (J1, J2).
10. Tuning Frequency Accuracy
Our accuracy will mainly be determined by the DDS used for the local oscillator
(LO). The DDS is stable because it's time-base is a crystal oscillator. Sometimes a
small trimmer capacitor Is added to the crystal circuit to fine tune the oscillator's
frequency. Frequency accuracy can also be tested using WWV or GPS.
To start out with I'm going to use my N3ZI DDS-2 and then later on an eBay
AD9851 DDS controlled by the Arduino Mega 2560 R3. M0XPD uses it for his
communications equipment oscillator(s).
When I use digital electronics in my receiver I will have to shield it very well or the
signals generated by the square waves will be picked up by the sensitive analog
circuits. I'll need to build the digital circuits in a box made of copper clad pcb
material soldered into an enclosure with signals coming out on RG316 shielded
cable.
Another possibility when I use my Arduino to control the internal functions of my
receiver is to use a breakout board from Adafruit using the Silicon Labs SI5351
which is an multi-output DDS oscillator. Depending on which one you use you can
have up to 8 different output frequencies all controlled by two wires from the
Arduino. I can use the smallest of the SI5351 Here's some links:
Planet Arduino - SI5351
Using the SI5351 with the Arduino (video)
Generate clocks with SI5351 and the ARduino
SI5351 Investigations, NT7S
11. Starting to Build the First Receiver
We'll need a place to work, tools and must decide which method will be used to
construct our circuits. There are several methods of construction including Ugly
construction, Dead-bug, Muppet and the conventional pc board. Examples of
these methods will be found through-out this book.
As stated before we'll start simple and work our way up. A simple 40m DC
receiver will consist of a simple tuned circuit input from the antenna into a NE602
mixer and then into a 4SQRP Group's Hi-Per-Mite audio section. We'll make a
simple local oscillator (LO) using the oscillator circuit built into the NE602. This
should yield an usable 40m CW receiver. So there will be some additional help
with this design we'll use most of the circuit from the 1st chapter of EMRFD.
ARRL provides chapter one online for free. The DC receiver starts on page 1.7 and
the circuit is on page 1.8 . At the bottom of page 1.8 you will find a great example
of how the tuning component values are calculated. Use those methods to
calculate out tuning components. To eliminate the band-spread capacitor I will
use the 8:1 tuning ratio 365pf cap. To bring its value down to the desired
capacitance I will put a 100pf capacitor in series with it and then use the same
inductance value in the example.
I think what I'll do is use a 120pf trimmer series capacitor, put the combo on my
trusty AADE LC meter and adjust it for the desired range. Note that in the
calculations we will have to allow for the two series 680pf caps of the colpitts
NE602 oscillator.
So, what we'll have to do is leave the two 680pf caps in the circuit along with the
270pf cap that couples the tuning circuit to the oscillator circuits.
The desired range is the 40m CW band from 7.000 to 7.150 MHz. With the
inductor as used in EMRFD of 1.16 uH I can calculate with the simple resonate
frequency formula the desired capacitance at the low and high end.
So using the standard formulas or a online calculator like found at
http://www.1728.org/resfreq.htm
At 7MHz and 1.16uH we need 446pf then at 7.150MHz we need 427pf.
Therefore, the range of capacitance is only 446 minus 427 or 19pf. So we'll have a
fixed capacitor of about 427 pf in parallel with the tuning capacitor that will only
vary 19pf.
Here's the schematic of the EMRFD DC receiver. My circuit will be the same
except the Hi-Per-Mite filter will be inserted between the NE602 circuit and the
LM386 audio amp.
Here's the Hi-Per-Mite from 4SQRP group.
This audio CW filter performs very well and does it without the "ringing" so often
found in audio filters. Please purchase your Hi-Per-Mite from 4SQRP group.
Please note that a 250K ohm pot can replace R11 and R12 for use as a volume
control.
We'll improve on that design in two ways. Our audio section will use the LM386
like in the schematic but we'll have the active audio filter between the N602 and
the LM386 to give us some better selectivity and make it a usable receiver. I'll
also simplify the oscillator circuit by using a simple air variable capacitor with an
8:1 gear reduction drive eliminating the need for a band spread capacitor and
associated components. The gear reduction air variable capacitor is available
from Midnight Science Parts P/N 356-8-1 .
We'll use the manhattan construction technique just in case you haven't set up
your pc board making equipment yet. The manhattan technique uses simple little
pieces of pc board as solder points. See Chuck, k7QO's instructions on this
technique. Here's the circuit we'll use:
12. IF Amplifier
The purpose of the IF amplifier is to take the desired signal from a mixer and
amplify it and filter it. Also automatic gain control (AGC) may be incorporated
into the IF section. Usually the AGC circuit also drives an S-meter (signal-strength
meter). In this book we'll look at using several types of amplifiers including the
MC1350P, the HYCAS (hybrid cascode) and for IF amps with no AGC we may use
the LT1253.
The famous HYCAS (hybrid cascode) IF amp will ultimately be used. It has 3 hybrid
cascode stages of amplification along with AGC and S-meter circuits.
I have a design for the crystal filters that I will use. I plan to etch a crystal filter
pcb that includes an input and output amplifier.
I've picked out the LT1253 video amplifier for the crystal filter input and output.
It's very important for proper crystal filter operation for the input and output
impedances to be as designed.
The LT1253 is typical of an opamp in that it has very high input impedance and
low output impedance. That means with at the crystal filter input I can use one of
the LT1253 amps with a resistor between the output and crystal filter to match
the filter's input impedance. Likewise, at the filter's output, because the LT1253's
input impedance is so high we can use another resistor to match the filter's
output impedance. The data sheet shows the output impedance at 11MHz to be
only about 2 ohms.
Notice in the schematic below the RC network with an output labeled 1/2VCC.
The LT1253 is not designed for single supply operation (not many opamps are)so
we need a voltage halfway between VCC and ground for the input(s) of the amp
to they're referenced half-way between the supply voltage like they would be if
there was a plus and minus power suppl.
I got the idea for this filter circuit from the SS-40 receiver circuit where Jim
Kortge, K8IQY, circuit needed 100 ohm input/output impedances for his crystal
filter.
My 6-crystal filter has an input/output impedance of 150 ohms. Consider the
output of the LT1253 to be at ground since it's output impedance is so low. Since
it's a video buffer it's output impedance is much lower than a typical opamp.
Therefore, the 150 ohm resistor in series with the output is connected to ground
level through the LT1253 output and yields an output impedance of 150 ohms.
At the output of the filter we have another 150 ohm resistor connected to 1/2VCC
which as far as the amp is concerned is ground. Then the filter output is fed into
the input of the 2nd amp which with its very high input impedance doesn't affect
the output impedance of the filter.
I found this schematic on
http://users.tpg.com.au/users/ldbutler/LadderFilter.htm after I had already
decided on the 6-crystal filter. It's designed for 11.5MHz where mine is 11.0592
MHz and it's designed as a SSB filter with a bandwidth of 2.18KHz. My CW filter
will look about the same except the capacitor values will be higher.
By the way, these circuits are perfect for using my Designer Dream boards from
4SQRP. The DD boards have place for a 4 crystal filter so I'll start out with that.
This is the filter that the DD boards are made for. Jim designed the board with
input/output transformers because he knew the importance of presenting the
filter with the correct impedance and terminating it with the correct impedance.
However, I am going to use the LT1253 as described above.
C11 = 150pf c19 = 270pf C21 = 270pf C12 = 150pf
C16 = 270pf C20 = 150pf C22 = 270pf C17 = 270pf
Notice that these capacitances are much larger than those for a SSB filter. The
larger the capacitors the narrower the filter. Of course you cannot just start
changing filter capacitors. instead you need to use the filter software to get the
proper values. The CW filter above was measured at 588Hz at 6db down.
I don't have a nice spectrum analyzer like this. This is from 4SQRP group's info on
the SS-40 receiver. I am, however, looking at the Rigol inexpensive spectrum
analyzer with tracking generator. EDN magazine compared it favorably to an
Agilent costing ten times as much.
Sept. 6, 2014 - Well, I now have a Rigol Spectrum Analyzer with a Tracking
generator and just learning how to use it. Not having had the luxuray of such a
piece of equipment in the past it will take me a little while to learn to use it. I
want to be able to measure fundamental freq along with harmonics and also
display the bandwidth of BPFs.
Crystal filter of the 4SQRP SS-40 IF filter.
This circuit is from the SS-40 receiver. I will insert the 6 crystal 500Hz CW filter
and change the resistors to 150 ohms.
AGC is another consideration in the IF section of the receiver. Basically, AGC (Automatic Gain
Control) reduces the gain of the receiver in the prescencs of very strong signals. If read that
AGC derived from the audio signals don't work as well as thoe derived from the final IF
amplifier.
An IF derived AGC does the following:
1. Extracts a sampling of the final IF signal without loading down the IF. That usually
means an AGC amp with a relatively high input impedance and enough gain to give us a signal
strong enough to process through the AGC signal processing circuits.
2. We need a DC signal that changes with the strength of the incoming signal that in turn
controls the gain of the IF amp stages as well as the RF amp in the front end.
3. Then we feed that DC signal into a series of RC networks and amps to give us a DC
voltage that changes exactly the way that will control the gain of the IF and RF amps the way we
want.
4. We then go through a "threshold" amp that establishes a signal level where the AGC
starts to take affect because we really don't want any AGC gain control at all until the incoming
signal strength gets up to a certain level.
5. We then feed the signals into the appropriate amps to control their gain.
A typical superhet receiver with IF derived AGC controlling gain of IF and RF amps. (source
2005 ARRL handbook)
An audio derived AGC system is very similar in operation but not as popular.
Audio AGC systems are usually those in more simple designs and for the sake of
simplicity and a smaller parts count.
13. Selecting the Proper Components
In this section we'll discuss selecting components for temperature stability. For
instance, the need to use NPO ceramics in frequency determining circuits.
The Spring 2009 QRP Quarterly has an article about choosing capacitors. I don't
know where you might get a copy of that article other than by purchasing a copy
of The Best of Idea Exchange , a collection from the QRP Quarterly.
For choosing a capacitor for a particular circuit here are some "Rules of Thumb".
- NPO/COG for all RF frequency determining circuits
- XLR next best
- Y5U/Y5V - ordinary capacitors, Use for everything else.
- For those 20% caps don't use unless they're electrolytic or tantalum in audio and
power supply circuits.
If you are buying capacitors for RF circuits its best to just purchase all NPO
ceramic types. NPO ceramics are usually marked "NPO" or in the case of disc
ceramics there is a black mark on the top of the disc.
A good rule of thumb is to purchase caps with a voltage rating 4 times that you
expect them to experience in the circuit.
What if you have an oscillator circuit that drifts badly with warm up? It's probably
a capacitor that is the culprit but how do you find out which one it is? With the
oscillator operating and listening to its signal in a receiver, blow though a soda
straw aimed at each capacitor until you find one that makes the frequency change
dramaticlly with its change in temperature.
A note about decoupling capacitors: Sometimes the inductance of a 0.1 bypass
capacitor begins to show up as the operating frequency is increased so a solution
is to parallel each 0.1uf with a 0.01.
14. Noise Blankers and Noise LImiters
Noise Blanker Design by N7WS
15. Construction and Building of PC Boards
I will use high quality IC sockets for the chips used. Since I've sort of mastered the
muppet pcb etching process I'll probably use it on almost all of there boards
except the HYCAS IF board which I was able to purchase several of. Another
technique I like is the "manhattan" style of construction. Here is a couple of links
to explain:
K7QO Manhattan Constuction
K7QO Advanced Manhattan Construction
I you use manhattan style of construction you will want to purchase some
MePads and MeSquares from QRPme.
I built a simple DC receiver using manhattan construction and you can find the
example elsewhere in this book.
Sometime its called "Pittsburg Style". See the 4SQRP regen receiver for an
example. The example shows it has round pads where mine has square pads but
its really the same technique.
I already have a couple of N2ZI super DDS2 working so I'll proceed with
connecting one of them to a NE602 mixer followed by a Hi-Per-Mite audio section
to yield a complete DC receiver. Well, not quite. I will need to add a BPF to the
front-end for the band I will be testing it on. (40m to start with)
I've recently had a great deal of success with the muppet style of making pc
boards and will show that method in detail here.
I've recently added to my garage mess a simple setup for making my own printed
circuit boards. This process is taken from instructions in the QRP-tech Yahoo
group headed by Chuck Adams, K7QO. Following Chuck's instructions I was able
to successfully make a small pcb 1st try. I picked out a simple circuit - a noise
generator that can be used for receiver testing.
The next muppet pcb I have made is for experimenting with a colpitts JFET
oscillator with an inductor and capacitor added to "pull" the oscillator frequency a
little bit (a VXO - Variable Xtal Oscillator). The QRP-tech Yahoo group is
experimenting with using the Fairchild and/or ON Semi J310 N-Channel RF
amplifier. The muppet layout below may not appear to scale. If you try to use it to
make a pcb make sure it's scaled to the proper size. The width is exactly 3.2" . I
purchase SS pcb material in a 4 x 6 inch size and try to make all my muppets half
that size. I always make an assembly drawing to use in assembling a pcb but even
more important is the placing of components on the assy dwg sometimes catches
pcb layout mistakes. An example of finding a layout mistake this way is illustrated
below. Notice on the assy dwg the connection between the capacitors and the
JFET source is drawn in as a line. That's because I accidentaly left it out. The layout
at the left is after I went back and added that connection. I layed out this pcb to
accomadate a nice little 10K trimpot. If you were using this circuit in a receiver
you would probably want to use a front panel mounted 10K 10turn pot.Notice
that the square pads that look odd are actually ground plane connections.
Expresspcb allows this kind of pad to thermally isolate the pad from the large
ground plane so soldering to it is much easier. Before using this layout also needs
to be flipped.
I purchase my SS pcb material in 4" x 6" size and cut it in half giving me two 3" x
4" pcb's. Then in the layout with Expresspcb I leave 0.4" all the way around to be
etched away to leave a nice looking "frame" around the outside edge of the
board. . .so, my 4" wide pcb layout becomes 3.2" wide (2 x 0.4 subtracted on each
side.). . .but, I still attach it to the 3" x 4" piece of material. For pcb's that are
intended to be installed in some piece of equipment you don't want to have that
cute little "frame" around it so that when you drill holes in the corners the metal
screw can ground the pcb to the metal enclosure.
FOUND ANOTHER MISTAKE If ya use this layout you must add a jumper between
the 100K resistor and the gate connection.
I got my image to print by printing from Expresspcb to a .pdf file. (If you don't
have that capability on your PC you can dowload cutePDF and add that
capability.) Then I open that .pdf file in photoshop, flip it and print it. If you don't
have photoshop I suggest googling the phrase "free photoshop" and you'll see
there are various ways to accomplish it. There are a multitude of free graphics
programs plus I believe Adobe will allow you to download and use Photoshop CS2
for free.
(One more modification to my layout. I added a few pads so I could add some
decoupling components to the drain connection if needed. Since it's such a minor
thing I won't show it here.)
_____________________
How to really mess up the making of a muppet pcb: January 31, 2014
The first muppet board I made came out perfect. I followed all the instructions
perfectly. But on the 2nd I got lazy. Here's the things I did wrong.
1. I chose some old pcb material that was 2oz copper instead of 1oz and it was
double-sided.
2. I didn't cover the other side requiring the process to etch many times the
amount of copper.
3. I did it in the garage (that's the correct place to do it) but it was 30 F outside
and the etchant was cold too. Remember, heat enhances chemical processes so
cold slows it down.
4. When I put it in the etchant I started stirring slowly but after a while got tired.
After 30min it still wasn't completed but some of the resist had come off so here's
the result.
It pays to follow instructions exactly, and do it in a warm
room.
___________________
Here's another muppet problem I created: Feb 14, 2014
I thought it'd be cool to use blue pcb material instead of that ugly green. Well, as
it turns out it creates a problem while you're etching the board. The etch resist is
black and the pcb material is dark blue so while you're etching it's difficult to tell
the difference. That is, when all the etching is complete the board will appear to
be completely black. It's better to use a light colored pcb material so you can see
the progress while you're in the etchant. If you really want to use the blue you can
use a HIGH-powered LED flashlight up close to monitor your etching progress.
It's best to follow Chuck's directions as outlined in his lab notebook.
I just finished a muppet pcb for the local Oscillator (LO) for the 40-40 transceiver
(xcvr)
_____________________
February 15, 2014. More mistakes. See the layout below.
Notice in the bottom left there are two ground pads that aren't connected to
ground.
Now it's fixed.
The finished board. You might see a little paper still on the black toner. If you can
see that it's NOT on the exposed copper it doesn't hurt anything. If there is a very
thin coating of paper on the copper it will be etched away with the copper so
don't worry about it.
_____________________________
This is the Ugly Weekender VFO board.
OOPS! Another mistake that is commonly made is to forget to flip the image so
that it comes out right on the final pcb.
___________________________________
Here's another muppet board that I made a mistake with. Notice the toner did
not adhere properly in places. Remembering back to the procedure to identify the
mistake, I think it was in washing the board to clean the board after scrubbing it
with the Scotch Brite. I used a lot of Dawn dishwashing soap and I'm pretty sure
the problem came when I didn't rinse the soap off properly. I think next time I
won't use soap. I mean, after all, the scrubbing with the Scotch Brite should get all
my finger prints off.
Notice the pads for the resistors and capacitors are fairly close together. It allows
you to use 1206 SMD parts if you want.
Here's a correct board.
. . .and, here's the final board.
Looks good !! I made it on thin, blue copper clad. the blue makes a nice looking
pcb. In the etching process you have to use a high intensitiy LED flashlight to
watch the process because the unetched areas are black and the pcb material is
dark blue and you can't tell when the etching is finished if you don't use the light.
BTW, I timed the etching process and it took 5min. I kept on etching 'til 6min was
up just to make sure. That was with me stirring the etchant continuously.
14. Crystal Filters
In this chapter we'll look a various types of crystal filters, how to design them
using readily available software and then provide an actual example of a crystal
filter we'll use for a CW receiver IF.
In this section I'll show my circuits and methods of selecting crystals for the IF
filters. I just purchased a few hundred HC-49 size Fox crystals at 11.0592 MHz. I
will pick out about 50 crystals, number them, measure their frequency and enter
each's exact frequency in an Excel database. With Excel I can sort by frequency
and easily pick out those that are very close, say, within 10Hz.
Please see my web page on filters at http://www.w5dor.com/W5DORCrystalFilters.html
17. Using an Audio Filter
In this section we'll talk about active audio filters. I used to think audio filters
were not much good until I saw the 4SQRP group's Hi-Per-Mite in action.
It's not uncommon the have problems such as ringing and other distortion with an
audio filter but the filter we will be using will be free of these problems.
18. Constructing the Inductors
There are several ways to construct an inductor including using toroid or bifilar
cores, winding your own coils with wire and a round insulator and just using of
the shelf inductors purchased from a distributor. Purchased inductors may also
be modified to meet reuiements. See IF Cans in the next section.
Most if not all of my inductors will be simple toroid inductors unless I use some
off-the-shelf IF cans. There are several toroid winding tutorials and selection
guides on my web at http://www.w5dor.com/W5DOR-Links.html . Scroll down
the right column to the section on toroids.
19. IF Transformers
I'm thinking of using 11.0592MHz for my IF frequency and if so I might have a
reason to use a standard 10.7MHz IF can.
20. The BFO
The BFO circuit will probably be a VXO and so arranged to be able to choose USB
or LSB. I will have it's frequency adjustable but also have switching to LSB and
USB.
21. Switching Various Modules
As the receiver evolves I will be adding multiple bands and will have to switch
front-end BPFs, LO band and IF filter widths. I plan to use small relays controlled
by the Arduino controller. I'm still trying to find the best all-around small relay
that doesn't required much current to activate and provides a good switch for
small signals.
22. Reducing Noise with DSP
There is a DSP DNR module available that I may include to further reduce noise.
See the sensitivity section above.
23. The Audio Amplifier
I want the audio amplifier to be able to drive any speaker that might be used so
I'll try to have just enough power to do so.
If you don't need to drive a speaker you can use a TDA7050, a stereo audio
amplifier that requires no external components.
The TDA1015 might be a very good one to use since it have an independent
preamplifier built in and has a hefty 4 watts output. I could use the preamplifier
into an audio filer and T/R FET switch and then into the power amplifier section.
I have an audio section designed but not built. It uses a TS922 opamp for a
preamp and a LM384, 5w amp for the output. (The TS922 is drop-in replacement
for the AD822 but with a little better spec's.)
This circuit is from the SS-40 receiver. I will insert the 6 crystal 500Hz CW filter
and change the resistors to 150 ohms.
24. Single-sideband Operation
The BFO section will be switched so that the proper BFO frequency will select the
desired side-band coming through the IF. I'll also have a variable BFO frequency
adjustment. I have an idea to have both USB and LSB switching of the BFO but
also have a position on the BFO switch for variable frequency that will be
adjustable with another knob.
25. The Power Supply
At first I will use a battery to help eliminate noise coming from the power supply
and then when the first receiver is working I'll add a power supply and therefore
will be able to tell if it's adding any noise.
I have a couple of 7.2VA 12VDC sealed lead-acid batteries I can use. I've also used
Mallory 12VDC lantern batteries.
For an AC power supply I plan to make it as noise free as possible. I have some
line filters for the AC input. I'll use a generous amount of by-pass capacitors, a
LM317 or LM 338 regulator and on the output a common-mode inductor.
26. The Enclosure
The first version of this receiver will be built in an old HW-32 enclosure. Another
possibility is some old 75A-3's that are essentially for parts.
27. Painting and labeling
I plan to use the old HW-32 front panel as a template to cut out a thinker
aluminum front panel. I want the enclosure to be black with the front panel a dark
gray - dark enough to use white labeling.
I also have a couple of old 75A-3's that i bought as junkers that are essentially 19"
rack enclosures. I can paint the enclosure and purchase a standard panel to
replace the front panel if I choose. There are several parts in the old 75A-3's,
however, that I plan to keep like the PTO and I'll use the S-meters for sure. I'll
save the tubes and offer them to hams engaged in vintage rig restoration.
28. Parts Sources
Almost all my parts are in stock in my "junk box" but I still search some of the
surplus retailers for bargains that might be useful. I use Mouser when I can. I've
been using Mouser since they started many years ago. They are close enough
here in Texas so that I can order anything up into the evening and get it the next
morning.
I've purchased several packages of capacitors and molded chokes on the internet
so that I have a complete set of all the values. I do, however, since most came
from China, check each with my AADE LC meter before using it. Before I had my
AADE LC meter I had a similar priced meter from a well known producer of
inexpensive test equipment and I found it to be rather inaccurate. The AADE LC
meter is the one to have.
I have some narrow boxes with envelopes with the ends cut off and labels writen
on them to sort the components. NPO ceramics, non-NPO disc ceramics, silver
mica capacitors, molded RF chokes are some that I have sorted.
Mouser Electronics
Digikey
Goldmine Electronics
MJPA
29. Keeping records
I guess keeping records is what I'm doing writing this book. Accounts of other
projects can be found on my web at http://www.w5dor.com . See the contents
list at the right.
30. Bibliography
Here I'll list my best books and sources of information. (I'll complete it later)
EMRFD (ARRL)
Stolid State Design for the Radio Amateur (ARRL)
QRP Quarterly
The Best of Idea Exchange (Like ARRL Hints & Kinks but from QRP Quarterly)
W1FB's QRP Notebook (ARRL)
W1FB's Design Notebook (ARRL)
QRP Classics (ARRL)
ARRL Handbooks (ARRL)
QRP Power (ARRL)
More QRP Power (ARRL)
QRP-tech Yahoo Group
EMRFD Yahoo Group
PHSNA Yahoo Group