gsmc - A GTK Smith Chart Calulator for RF impedance matching
gsmc is a GTK application that allow to do all the calculation usually
done on a Smith Chart. It permits to make calculation for network
composed by resistor, capacitor, inductance, and trasmission line (also
as stub). Network topology is limitated to series, parallel and as
trasmissive for trasmission line. No series connection are allowed in
parallel branch, neither parallel connection in series branch.
Trasmission line can be placed as quadrupole or as a parallel or series
stub, either opened or shorted at the other end.
Calculation procedure starts with setting the initial impedance to be
matched, next network elements are added and tuned to obtain the
desidered impedance value.
The network so obtained can be saved in spice format for other
analisys; current work can be saved for succesive retrieve.
The initial (start) impedance is thought as "the load" so when adding a
trasmission line placed as a quadrupole rotation is clockwise, going
"toward generator"; generator is placed after the last network element,
as can be seen from spice output.
gsmc is a menu based GTK application with as much as possible
accelerators and mnemonics. Nearly all the command can to be issued as
a single keystroke or keystroke with modifiers. This is, in the
author’s opinion, the preferable way for very specialistic programs
that has to be used extensively.
Ctrl+q Quit the program.
Ctrl+1 Toggle visualization of impedance (Z) circle
Ctrl+2 Toggle visualization of admittance (Y) circle
Ctrl+3 Toggle visualization of reflection coefficient (RHO) circle
Ctrl+4 Toggle visualization of constant Q circle
s Set start point: can be entered as impedance, admittance or
Set characteristic impedance z0
f Set frequency (f0)
r Add a resistor
l Add an inductor
c Add a capacitor
t Add a transmission line or stub
Insert a resistor before the highlighted element
Insert an inductor before the highlighted element
Insert a capacitor before the highlighted element
Insert a transmission line or stub before the highlighted
d Delete the currently highligthed element
Highlight previous element
Highlight next element
Decrease the first field of currently highlighted element
Increase the first field of currently highlighted element
Decrease the second field of currently highlighted element
Increase the second field of currently highlighted element
Increase the tuning step
Decrease the tuning step
Ctrl+w Write network file in spice format
Ctrl+n Restart for a new calculation with a clean chart
= Change first field of currently highlighted element
Mod+= Change second field of currently highlighted element
Ctrl+a Start autotune procedure (tune goal is characteristic impedance)
Ctrl+x Toggle lock flag in autotune procedure
Ctrl+p Write an EPS or PS file
Ctrl+l Load a previously saved .B gsmc job
Ctrl+l Save a .B gsmc job
Ctrl+f Increase frequency
The single window of gsmc is divided in four parts:
- The Smith chart itself in the upper left portion, where arcs and
constant circle are drawn.
- The vertical bar on the rigth of the Smith chart, where initial,
final and cursor impedance are presented togheter general data.
- The horizontal bar on the bottom of Smith chart where the
network element are listed.
- The portion in the lower right portion, for future use...
EXAMPLE OF USE
Let’s try a simple example to understand how gsmc work. Suppose to have
a load of 10-j35ohm to be matched to 50+j0ohm at an operating frequency
of 15.5MHz and we can use only concentrated reactive elements, i.e.
inductances and capacitors.
So start with setting operating frequency: press ’f’ and a dialog win
will appear. Type in the string ’15.5M’ and then press enter. Now in
the vertical bar the frequency entered is displayed.
Next set the start point impedance: press ’s’ and a dialog win will
appear with the title "Start Point"; select the format for start point
either clicking the "Z" radio buttono or using the menmonic via Mod+Z.
Now the impedance can be entered as the string ’10-j35’ followed by
enter. The starting point is now displayed as a small circle.
The network has now to be constructed: let start with a series
inductance of 800nH, so press ’l’ and next introduce the value as
enter. TAB and Shift+TAB is usefull to move across items in dialog win
without leave hands from keyboard. This value is really too large, use
Shft+Left cursor to reduce a bit. Up to where? Let’s help you displaing
the admittance constant circles pressing "Ctrl+2", the arc should
arrive to the G=1 circle (also called the mirror circle), it’s arround
Now add a capacitor in parallel connection, saying of 200pF: press ’c’
and insert the string ’200p’ and select Parallel either with mouse or
with "Mod+p". Now the admittance circles may confuse, remove it by
pressing "Ctrl+2" again. The value of 200pF is too small, enlarge it by
pressing Shift+Left cursor up to get the end point near to the chart’s
center. The point reached with 429pF has nearly zero imaginary but is
43ohm of real part instead of 50ohm.
So back to the inductance and try to modify it, with Shift+Cursor up
move to the first component and then try to change the inductance value
by pressing cursor left and right. The changes are too large and 50+j0
cannot be reached. So press "Mod+Cursor down" and the "tunestep" value
shown in the vertical bar is reduced from 10% to 5%, try again to use
Shift+Curors left and right. It feel better. Now do the same on the
capacitor, moving to it by pressing cursor down and than adjusting
value as for the inductance.
Final values should be arround 568nH and 408pF. Now you want to see
what appen using the closest standard commercial values, that is 560nH
and 390pF in the E12 series. Select (with Shift+Cursor Up), if needed,
the inductance and then press ’=’, the dialog window appear (middle
line is unused) and the string ’560n s’ has to be introduced. Next move
to capacitor (Shift+Curs Down) and pressing ’=’ here insert ’390p p’.
The final result is of SWR=1.14, if it’s enough for you...
Moving the mouse to the point where the two arcs ends you can see that
the point has an impedance of about 10+j20ohm.
Trasmission line and resistor can be used similary refer to the section
INTERACTIVE COMMANDS for knowing how they can be placed. The major
difference with transmission line is that they have two parameter, the
electrical lenght and the caracteristic impedance: the former can be
adjusted with Shift+Cursor left/rigth as for resistor, inductance and
capacitor; the latter can be adjusted with Mod+cursor left/right.
Now the network can be saved either in a gsmc format (.gdt extension)
by pressing Ctrl+s or in spice format pressing Ctrl+w: The file so
generated can be now analized with a spice simulator or gnucap or
ngspice ; maybe you are interested in simulating it sweeping arround
15.5MHz, so modify the line
.ac lin 1 15.50MEG 15.50MEG
.ac lin 51 10MEG 30MEG
and run spice or gnucap
This example should be included in gsmc package an can be retreived by
pressing Ctrl+l and selecting example1.gdt.
Autotune algorithm is very primitive and may not reach what is very
simple to do by hand, do not ask to much from it, work in progress...
Charts so drawed can be printed to a Postscript or Encapsulated
Postscript file; in printed file some information are added on the
chart drawing as well as Circuit Description and some of the Current
Value are transcripted. Automatic recognition of filename extension
.eps or .ps allow to specify if the file has to be in encapsulated
format or not.
gsmc don’t use special configuration file, a gtkrc file is supplied and
can be tuned to user preferences; if present it must be in
/root/.gsmc/gtkrc. File describing the network currently analized can
be generated, it’s only needed that a minimum file permission is
allowed, if not the program will work but data could not be saved.
Data file (.gdt) contain a copy of internal data structures in a quite
human readable format; by inspection one can recognize and modify some
parts using a text editor. For more details compare a .gdt file with
smcdata struct defined in source file main.h.
Porting can be very very difficult, since all is based on X windows
programming and GTK library, but for other platform a plenty of similar
program are available, free and good one too.
Help me to find.
· Improve autotune algorithm
· Target settting (not alwais one want to match to z0) for autotune.
· Auxiliary window for error reporting instead of stderr.
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2 of the License, or (at your
option) any later version.
Lapo Pieri (IK5NAX)
Home address: via A. dei Corbizi 9 I-50127 Firenze Italy
Phone: +39 055 410209
e-mail: email@example.com, firstname.lastname@example.org
spice3(?), gnucap(1), ngspice(1)