Practical Application of DDS

Transcription

Seminar in
Broadband HF transceiver
design experiences
Multimedia Room, Polivalente II, 2nd floor
Campus de Tafira, Las Palmas
Speaker: Baltasar Pérez Díaz <[email protected]>
16th May, 2014
www.idetic.eu
Author: Baltasar Pérez Díaz
Broadband HF transceiver design experiences
BACKGROUND INFORMATION
Broadband digital HF transceiver
DUC/DDC
multichannel
(FPGA)
1KW Power
amplifier
Driver
Wattmeter
Broadband
Transceiver
More than 15 years developing HF modem (software) (IDeTIC+UPM)
Last 10 years hardware development
HFDVL (High Frequency Data+Voice Link) architecture
1MHz bandwidth (not signal) = +300 times conventional radio (3KHz)
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PRESENTATION INDEX
INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
1. HF broadband digital transceiver introduction
a. Block diagram and pics
2. Main problems and solutions
a. Local oscillator (DDS spurs, reference clock)
b. Filter (low availability, limited)
c. AGC (algorithm and narrow band interference)
d. Amplifier (intermodulation products)
e. Antenna (broadband antennas)
3. Summary
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BROADBAND DIGITAL HF TRANSCEIVER INTRODUCTION
Receving section block diagram
INDEX
INTRODUCTION
Receiver
antenna
STATION
IF to be
digitized
MAIN PROBLEMS
AND SOLUTIONS
RF receiver
(analog)
A/D
Converter
Digital
receiver (DDC)
Signal
processing
D/A
Converter
AUDIO
SUMMARY
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•
RF-Front end moves 1MHz to IF (10.7MHz). Implements a double conversion
superheterodyne architecture based on DDS
•
A/D D/A converters and DDC/DUC : Altera FPGA (Cyclone II EP2C70)
•
Signal processing: Fixed point DSP (TMS32C6416) and software running
on a GNU/Linux PC.
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RF Front-end and FPGA setup
INDEX
INTRODUCTION
FPGA
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
RF Front-end
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RF Front-end block diagram
INDEX
INTRODUCTION
To FPGA ADC
Receiver
Antenna
MAIN PROBLEMS
AND SOLUTIONS
Relay
SUMMARY
Oscillators
Control
Unit
Transmitter
•
•
•
•
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From FPGA DAC
Double conversion superheterodyne architecture
Half-duplex transceiver: two common blocks
Oscillators based on DDS (v0), later DDS+PLL (v1)
Control unit: manages front-end and interacts with operator using a PC
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Receiver block diagram
INDEX
INTRODUCTION
Antenna
3-30 MHz
MAIN PROBLEMS
AND SOLUTIONS
ADC
Band Pass
Filters 3-30MHz
SUMMARY
RF Amp
Image-rejection
Filter 3-30MHz
BW = 1.0 MHz
112 MHz
≤ 20
dB
BW 1 MHz
10.7 MHz
≤ 75 dB
Low Pass
10.7 MHz
82-109 MHz
DDS
122.7 MHz
µController
CARDS12
AGC
RS232
•
•
•
•
•
•
•
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Double conversion superheterodyne (up-mixing).
1st IF: 112MHz and 2nd IF: 10.7MHz.
Frequency step: 1Hz.
Sensitivity (@1MHz): -70dBm.
Microcontroller-based RX, TX, AGC and ALC control.
Modular assembly.
TX output power +20dBm.
External power amplifier (linear class A) 5W.
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
ANT
RX OL
TX
Control
PS
19’’ subrack
3U form board (100x160mm) with connector DIN41612
Rear: digital signals
Front: analog signals
Power Supply: 220VAC and 12VDC
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MAIN PROBLEMS AND SOLUTIONS
INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
3. Summary
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INDEX
•
•
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
DDS (Direct Digital Synthesis) ⇒ is a method of producing an analog
waveform,usually a sine wave,
by generating a time-varying signal in digital form
• converted into analog signals using a DAC
•
DDS is a standard in radio equipment oscillator.
•
NCO (Numeric Controlled Oscillator) ⇒ also called
•
Advantages
SUMMARY
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•
•
•
•
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Capable of generating a variety of waveforms (sine, triangle, square)
Preferred form of signal generation nowadays
Fast switching capability (freq. hopping systems (phase-continuous))
High precision ⇒ sub Hz (mHz) and sub degree phase tuning
Digital circuitry
• Small size (single chip) ⇒ fraction of analog synthesizer size
• Fewer components per system - low cost
• Small low-powered devices – portability
• Easy implementation (no Barkhaussen criterion, PLL (LPF design))
• Fewer assembly operations / reduced product reject rates
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INDEX
Typical DDS Architecture
Digital Circuits
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
Sine
Accum
W Lookup R D-to-A
N
to
10 to
Fr 24 to -ulator 1416bits
48bits
Table 14bits Conv.
Low
Pass
Filter
f F
fout= clk r
2N
Sine
Wave
SUMMARY
1/fout
1/fclk
1/fout
1/fclk
1/fout
1/fout
1/fclk
• Sampling theory (sinc(x))
• Nyquist: Fundamental signal <= Fclk/2 (1/3 better)
• Filter required to eliminate unwanted products
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Broadband Transceiver oscillator board (v0)
INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
OL 1
SUMMARY
DDS 1
ANT
Clock
RX OL
TX
Control
DDS 2
PS
OL 2
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INDEX
Spurs !!!
Worst problem in DDS
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
•
•
•
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AD9835 (fclk=50MHz)
Spurs situation and quantity depend on output frequency (become birdies)
Mix up AGC
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INDEX
Spurs !!!
Worst problem in DDS
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
•
•
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AD9835 (fclk=50MHz)
Spurs situation and quantity depend on output frequency (become birdies)
Mix up AGC
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INDEX
Another problem: Clock reference purity
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
Comparison: frequency output purity 400MHz Clock vs Agilent Generator
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INDEX
Another problem: Clock reference purity
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
Comparison: frequency output purity 400MHz Clock vs Agilent Generator
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INDEX
Spurs sources
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
• A DDS have four principal sources of spurs. Most of them are predictable.
• Reference clock: highest purity and freq. possible. Spurs next to carrier.
Must have constant amplitude to avoid spurs.
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Analog Devices solution:
INDEX
SpurKiller Technology: The Results on a DDS Output Spur
INTRODUCTION
AFTER
BEFORE
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
500 kHz / DIVISION
500 kHz / DIVISION
OUTPUT FREQUENCY = 166 MHz
Fclk = 500 MSPS
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Analog Devices solution:
INDEX
SpurKiller Technology
INTRODUCTION
•
MAIN PROBLEMS
AND SOLUTIONS
Use an auxiliary DDS channel to add in a signal at the same frequency and
amplitude as the spur, but 180° out of phase with the highest spur… It’s all in the
Digital
Domain!
DDS Channel
for spur reduction
SUMMARY
Phase
Offset
Frequency
Accumulator
Σ
Σ
DAC
COS(X)
FTW
16
Register
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14
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DDS Channel
for phase
modulation
•
Spurs are therefore predictable.
•
In addition, the relative phase of each spur does not change.
IDeTIC Seminar
Register
DDS Channel
for amplitude
modulation
AD9911
DDS core
Register
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INDEX
Adopted solution was: Change synthesizer topology
DDS Used as PLL Reference
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
Phase/
Frequency
Detector
SUMMARY
Fref
Loop
Filter
VCO
DDS
RF
÷N
M
FRF = NM
Fref
2n
Take advantage of PLL low phase noise and spurs, and DDS frequency
resolution
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INDEX
Broadband Transceiver oscillator board (v1)
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
PLL: NB4N441
DDS: AD9835
SUMMARY
PLL
TCXO: 50MHz
DDS
• DDS output: 10 to 13MHz
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INDEX
Comparison: oscillator board v0 (green) vs v1 (blue)
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
Phase noise improvement: 20dB
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INDEX
PLL output: Improves
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
3. Summary
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
•
•
•
Limited filters with 1MHz bandwidth
Only found TFS112 (fc=112.32MHz, BW(3dB)=1.1MHz, L=12.5dB)
SAW (Surface Acoustic Wave) filter
• Low S11 (high mismatch) BRICK!! (Z=3.5Kohms)
• High attenuation
• 40dB band-stop
• 2.55 form factor
• Redesign TRX OLs
Measurement SAW Filter bandwidth
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
3. Summary
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
•
•
•
Problem: Big/good ears in a noisy environment (HF band)
Point 1) AGC algorithm must keep constant 1MHz varying signals
power at ADC input (few bits left for weak signals)
Point 2) Narrow band interference (NBI) intentionally (jamming) or
unintentionally (Broadcast radio stations) within signal or BW
NBI demo video
Two HF broadband TCVR 10 meters apart tuned to 14MHz freq.
Function generator as jammer (pure tone)
Taking output at BaseBand (10.7MHz) RF Front-end receiver
Using a Agilent VSA (Vector Signals Analyzer) 89600S
Showing 700KHz around 10.7MHz
Innovation #8 of the Top 10 Most Wanted Wireless Innovations
(Interference Mitigation Techniques) Software Defined Radio and
Cognitive Radio (CR)
Source: Wireless Innovation forum
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INDEX
•
Wireless Innovation Forum proposed solutions
(They are already used in high-end systems but want lower cost and widespread)
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS

Power control throughout the communications system
only enough power to maintain communications at the minimum acceptable level.

Adaptive beam-forming to maximize antenna gain in the direction of the
communication path and minimize gain in the interference direction.

Adaptable data rate to the minimum rate needed for the communication.

Adaptive frequency control to increase frequency separation of the interference
sources from the desired radio path.

Adaptive receive filtering to provide better rejection of the interference balanced
against possible sensitivity loss.

Improved roaming algorithms
(change sites or systems to one that has a better SINR).

Change channel coding algorithms to relax the required signal to interference
plus noise ratio at the expense of more data overhead.
SUMMARY
Some of them are difficult to implement at HF (heavier and larger antennas,
noisy environment, worldwide coverage, propagation)
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INDEX
INTRODUCTION
•
Our proposed solution
Interference must be mitigated enough to let the ADC work under proper condx.
Cancellation process after AD conversion (digital filtering)
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
Two phases: detection (SW) & mitigation (HW)
Detection: Zero crossing and Compressive Sensing (Random Demodulator)
Mitigation: Variable notch filter (BW3dB 50KHz; BW50dB 25KHz; Tunable 10.2~11.2MHz)
Bandpass filtering (xtal), LPF+HPF (need Q > 200)
technological challenge: low Q components (filters), diff. tunable and speed processing
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INDEX
•
Our proposed solution (cont)
Desired response
INTRODUCTION
Low Pass Filter + High Pass Filter
A(dB)
MAIN PROBLEMS
AND SOLUTIONS
3
50
LPF
BW=50KHz
BW=10kHz
SUMMARY
HPF
f(MHz)
10
Wien-Robinson active filter
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Sallen-Key 2nd order
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INDEX
•
Our proposed solution (end)
Double T
Band-reject LC filter
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
Schematic
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BW measurement
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
3. Summary
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INDEX
INTRODUCTION
•
Problem: Broadband signals on nonlinear amplifier
OFDM Signal PAPR (~10dB)
MAIN PROBLEMS
AND SOLUTIONS
- Peak to average power ratio (PAPR)
is one major drawback of OFDM.
SUMMARY
- Requires power amplifier (PA) to be
operated in the linear region, resulting
in poor efficiency.
- 1KW PA operating at 100W
- Commercial power amplifiers (PA)
have nonlinear characteristics and
need to be linearized.
Innovation #5 of the Top 10 Most Wanted Wireless Innovations
(Techniques to minimize power amplifier spectral regrowth in
non-continuous spectral environment)
Source: Wireless Innovation forum
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
•
Increasing gain (spectral regrowth)
(cont)
16 tones 25KHz spaced (400KHz)
SUMMARY
- Careful design to avoid intermodulation
products (below 1dB compress point)
- Poor effiency
- Dirty spectrum
- Interference to other band users
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INDEX
INTRODUCTION
•
(cont)
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
SPE Expert 1K-FA
800W
Amateur equipment
Class AB
Some segments in HF band
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E&I A1000
1000W
Professional Equipment
Class A
300KHz - 35MHz
low HD & IMD
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
•
(end)
Some techniques to overcome distorsioned PA ouput:
PREDISTORSION
SUMMARY
- adds harmonic content at phase
angles that cancel out the spurs
that the nonlinear RF PA creates
- allows driving RF PA closer to
saturation, which improves
power efficiency.
PAPR REDUCTION TECHNIQUES
- Clipping and Filtering: Let the signal clip and filter out of band signal
- Coding schemes: Try to avoid the case with all the subcarrier with same phase.
-…
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INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
3. Summary
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INDEX
Problem: Most of antennas have a very narrow bandwidth (10% fc)
27MHz (higher f more BW)
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
Transmitting 1MHz BW signal at 10MHz (fc)
SWR Curves
Filter behaviour (S11)
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INDEX
Solution: Fatter elements (CAGE DP), introduce losses (T2FD) or
INTRODUCTION
•
•
Cage dipole
x2 broader BW compared to dipole
•
•
•
•
•
•
T2FD (Tilted Terminated Folded Dipole)
30% RF Power converted heat at resistor
-3 to +3dBi gain
Modest size and cost
No need electronic matching
Used by Army and Civil Guard
MAIN PROBLEMS
AND SOLUTIONS
SUMMARY
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Solution (cont.): or Log Periodic antenna
INDEX
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
•
•
•
can operate on a wide frequency band (10:1)
has the ability to provide directivity and gain (4-8dBi)
radiation and impedance characteristics repeated as a logarithmic function of freq
SUMMARY
•
•
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NBI: different polarization
R&S HE016 antenna
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SUMMARY
INDEX
Remember: Broadband system introduces a number of challenges
INTRODUCTION
MAIN PROBLEMS
AND SOLUTIONS
•
It needs to coexist with traditional narrowband systems
•
So need to tackle problems as sensitivity to strong narrow sigs (AGC
mute)
•
Take into account the low linearity of commercial PA (IMD and interferes
other users).
•
Necessary to have broadband antennas capable to work with these sigs.
SUMMARY
Future Trends
16th May
•
Harris 25KHz bandwidth
•
New HF broadband SDR transceiver 25KHz with same form factor as
HFDVL modem. TX using FPGA, RX analog front-end+FPGA
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Seminar in
Broadband HF transceiver
design experiences
Thanks for your attention!!
Any question?
Multimedia Room, Polivalente II, 2nd floor
Campus de Tafira, Las Palmas
Speaker: Baltasar Pérez-Díaz <[email protected]>
16th May, 2014
www.idetic.eu
Author: Baltasar Pérez Díaz