QuickSmith

QuickSmith
by Nathan Iyer
Users Guide
Introduction
QuickSmith is a Smith“ Chart based radio frequency circuit simulation program for Microsoft
Windows‘. QuickSmith was designed to perform a rapid and efficient Smith Chart calculation.
It uses the advantage of Windows environment to provide friendlier way to simulate Graphical
RF designs.
Salient features
Some of the features offered by this program are listed below:
- Ladder network, elements are loaded using drag drop method.
- Open and Save functions for schematic files
- Impedance matching
- Frequency/component sweep
- Load impedance interpolation/extrapolation for frequency dependent loads
- Q factors taken into account for network components
- Amplifier design/analysis using gain/noise circles
- Insertion loss and S21 graphs
- Transmission line parameter calculations
- Constant Conductance, VSWR and Q circles
- Import and Export of data files
- Help files with solved examples of network matching and amplifier design
This program is ideally suited for students as it will help to double check calculations and make
Smith chart more interesting. If you have any suggestions or find any bugs please let me know, I
will make every effort to correct and send you a fresh copy.
I hope you enjoy the program.
Installation
To install the program insert disk in your floppy drive A (or B); select Run from your Windows
Program Manager; then type A:\SETUP (or B:\SETUP). The setup program will request the
directory to install, choose enter or type in a directory name.
The installation program will create a program group and will install 5 icons which is listed as
follows.
1) QuickSmith - Main executable program
2) Gsw.exe - Graphic Server
3) Exm.hlp
- Examples in Windows Help file
4) Qs.hlp
- Contents in Windows Help file format
5) Readme.exe - readme.txt
Double click the QuickSmith Icon to start QuickSmith
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QuickSmith Program Description
QuickSmith consists of two basic windows, namely: Smith Chart (or Chart) window and
Schematic window. The Chart window displays a Smith Chart with a few circles of constant
resistance and reactance which are labeled with normalized values. A blue dot on the chart
indicates the current position of the input impedance whose values are displayed on the top left
side of the window. The characteristic impedance appears at the bottom right of the chart. The
blue text box on the top panel shows impedance of any point on the Smith chart pointed by the
tip of the mouse icon, more information about the point can be obtained by double-clicking the
left mouse button. In the Chart window there is a child window called Tuner it allows to
remotely tune the elements.
The window size can be changed by using the window menu or by simply stretching the window
panes using the mouse. Access to more functions can be obtained by using the Smith Chart menu.
Figure 1A
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Figure 1B
The Schematic window can be obtained by clicking the tool-icon. This window consists of a
ladder network terminated with a load on the left side. On the right most side the impedance
values in ohms is displayed. At the bottom of the window there are various component icons.
Gamma, Resistor , Reactance, Capacitor, Inductor, Series Transmission Line, Null, Open Stub,
Short Stub, Shunt Parallel-Tank, Shunt Series-Tank, Series Series-Tank, Series Parallel-Tank
Circuit components are assigned by dragging the icons and dropping them on the ladder
slots.
Gamma icon is used only for load, when the Termination Menu is set to Gamma.
On the top portion of the window there is frequency box displaying the frequency in MHz ,
Insertion loss of the network in dB and length units of the transmission lines.
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QuickSmith Methods
i) Multiple Windows
Schematic and Chart Windows can be tiled horizontally, vertically or cascaded by using Window
menu.
ii) Tuning Components
The scroll arrows adjacent to the value box are used to tune the variables. The step size can be
changed by double-clicking on the value box. The tuner box on the Chart Window may also be
used to tune components.
iii) Drawing Admittance Q and VSWR circles
Admittance circle can be drawn by clicking the Show Admittance Item in the Display menu of
the Chart window. These circles are very useful for tuning shunt elements. The Show Admittance
item in the menu is checked when constant circles are drawn.
VSWR and Q Circles can be drawn on the chart by incrementing/decrementing the scroll arrows
adjacent to the value box as per the Step Size. More than one circle can be drawn in a chart.
A Redraw Chart command or a window paint message clears all but the most recent circle.
The default value of the VSWR circle is 1 and the default value of the Q circle is 0.
The step size can be changed by double-clicking on the value box.
iv) Setting up Marker
Marker can be set using the ShowMarker Item in Display menu of the Chart Window. This
toggle command places a blue marker (a circle with a plus sign in the center) on the Smith chart.
Before it draws a marker the user is prompted to enter the impedance values for the marker data.
This mark is useful for impedance matching when the target impedance is not at the center of the
chart.
The Marker Item is checked when marker is drawn. To turn the Marker off choose the set
ShowMarker item again.
v) Parallel/ Series Conversions
Series/Parallel equivalent circuit of the current impedance values modeled as a resistor in
series/parallel with a capacitor or inductor is displayed using the Parallel/Series Conversion Item
in Display menu of Chart Window.
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vi) Transmission line Equivalents
Transmission line Item of the Display menu displays a message box showing the Transmission
Line parameters of the network. The following is the list of different parameters shown:
i) Impedance in terms of Real /Imaginary and Magnitude/Angle.
ii) Admittance in terms of Real /Imaginary and Magnitude/Angle.
iii) Reflection Coefficients in terms of Real /Imaginary and Magnitude/Angle.
iv) S21 in terms of Real /Imaginary and Magnitude/Angle.
v) Voltage Standing Wave Ratio (VSWR) which is the ratio of maximum to minimum amplitude
of voltage along a wave guide or network.
vi) Return loss in dB which is the ratio of the power in the incident and reflected waves.
vii) Insertion loss in dB which is the dissipative loss due to the network or wave guide.
viii) Reflection loss in dB which is the nondissipative loss introduced due to mismatched
termination.
ix) Transmission loss is the ratio of combined dissipation and radiation losses in a wave guide or
network when mismatch-terminated and match-terminated.
x) Maximum value of the standing wave.
xi) Minimum value of the standing wave.
vii) Admittance Equivalent
Admittance equivalent can be displayed by choosing the appropriate Y-in or Z-in option located
on the top left of the Chart Window.
viii) Gamma / VSWR of any point
Gamma and VSWR values of any point are displayed on a message box by double clicking the left
mouse button. To increase graphical precision maximize the Smith Chart Window.
ix) Reset Chart or Schematic
The New chart item in Display menu of Chart Window resets the chart data and hence can be
used to initialize the sweep trace.
This New item of the File menu in Schematic Window initializes the schematic editor. The
existing schematic is erased and replaced with a short on the series slot and a open on the parallel
slots.
x) Converting Reflection Coefficients (Gamma) to Impedance
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From the "Termination Menu" in the schematic chart choose Gamma. Drag "Gam" tool on the
load and enter the gamma values. To convert to impedance, choose impedance from the
"Termination Menu".
xi) Converting Impedance to Reflection Coefficients (Gamma)
From the "Termination Menu" in the schematic chart choose Impedance. Drag "Resistance and
Reactance" tool on the load and enter their corresponding values. To convert to Gamma, choose
Gamma, from the "Termination Menu".
xii) Performing Sweep
The sweep function is very useful in inspecting the broad band capabilities of the circuit. It
allows the user to tune any circuit component and observe its effect on the entire circuit for a
specified range. The Sweep element, Start, Stop And Step Size can be entered using the “Sweep”
menu in the Schematic window. Maximum number of Sweep points is one thousand points. The
units of Start, Stop And Step Size is the same as the sweep element’s units. The actual sweep is
performed using the “Sweep” menu of the Chart window.
xiii) Import/Export Data into QuickSmith
External data from a file can be viewed on the Smith chart by using the import command on the
"File menu" of the Chart Window. The data format can be obtained from the sample files
provided with the program. Swept data can be saved using the Export command. Alternately the
user can also enter data directly from the keyboard by using the QuickSmith editor.
xiv) Non-Linear Loads
In many practical situations the "load" is frequency dependent. For example, a dipole antenna
used as a load for a broad-band design. QuickSmith takes this into account by allowing the user to
link the frequency-dependent termination to the program. The impedance corresponding to the
intermediate frequencies are computed using interpolation algorithms. The data can be linked
using Termination Menu in the Schematic Window.
xv) Insertion Loss Calculations
Insertion loss or S21 ( when load is equal to characteristic-impedance) can be calculated by using
the S21 menu in the Schematic window. Q can be taken into account through the “Assign Values”
menu. Once the sweep details are entered the user can click the "Begin Sweep" item or the S21icon to display the S21 Magnitude[dB] and Phase[Degree]. The output is displayed by a
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rectangular chart or by tabulated numeric data that can be also be saved in a file. Hard copy of the
output can be obtained by using the print command.
xvi) Amplifier Design/Analysis
S-Parameter based amplifier design/analysis window is opened by clicking the “Amplifier
Design” menu in the Chart window. After entering S-parameters, stability regions can be
calculated and plotted along with constant gain and noise circles, enabling the user to optimize
load and source impedance. The design can be matched by transferring the impedance to the
Schematic window via the transfer menu.
For more details see Examples in Help Menu.
xvii) Change the Characteristic Impedance (Zo)
Transmission lines can have different Zo values. Characteristic Impedance of the chart can be
changed from the Assign Values Menu in the Schematic Window. The default is 50 Ohms.
Solved Example
Finally an example from Motorola's application note AN721, illustrates a simple impedance
matching problem. In this example input and output matching network for a RF power transistor
is designed using Smith Chart.
Problem: Design a network to match 2N5642 to deliver 20W between 50 ohms source and load
at 175 MHz. Available supply voltage is 28 Volts. Design for a maximum input Q of 10.
Solution:
i) Input Match: The data sheets for the 2N5642 transistor provide input impedance information.
The series input impedance for the required operating condition is 1.94 ohm in series with a
reactance of 1.1 ohm. This value is assigned to the load by dragging the resistance and reactance
component and dropping them on the load-slot in the ladder network. The frequency is set to
175 MHz using the "Assign Values" menu in the Schematic window. The Q circle of 10 is drawn
from the Chart window. In the Schematic window we start with a series inductor to the load.
Since the input Q is specified at 10 we increase the inductance until the Q value of 10 is reached.
The next step is to add a parallel capacitor and find its value so that the input impedance meets
the 50 ohms constant resistance circle. Finally, we cancel out the remaining inductance by adding
a series capacitor to get to the center of the chart. The figure below illustrates the
transformations.
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Figure 2a
ii) Output Match: The required output power is 20W at 28 Volts supply, therefore the
collector resistance is equal to : (Vcc - Vce(sat))^2 / (2 x Pout) which is evaluated to be 15.6
ohms. The constant conductance circles are overlaid by clicking the "Show Admittance Circles"
item of the chart menu. The collector capacitance, which is in parallel to the load given by the
data sheet is 40 pf. This is equivalent to a resistance of 10.6 ohms in series with a reactance of 7.3 ohms. The figure below shows the Smith chart of the output circuit designed in a similar way
as the input circuit.
Figure 2b
References
[1] Phillip H. Smith, Electronic Application of Smith Chart, Robert E. Krieger Publishing
Company, Malabar, Florida.
[2]Guillermo Gonzalez, Microwave Transistors Amplifiers: Analysis and Design, Prentice-Hall,
Inc., Eaglewood Cliffs, N.J. 07632. Appendix B, UM-MAAD.
[3]Dale Henkes, An S-Parameter Based Amplifier Program, December 93, RF Design.
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[4]B. Becciolini, Impedance Matching Networks Applied to RF Power Transistors, Motorola
Application Note AN-721. Section 4.2 Graphic Design.
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