gsmc(1) A GTK Smith Chart Calulator for RF impedance matching




gsmc is a GTK application that allows one to do all the calculation usually done on a Smith Chart. It permits one to make calculation for network composed by resistor, capacitor, inductance, and transmission line (also as stub). Network topology is limitated to series, parallel and as trasmissive for transmission line. No series connection are allowed in parallel branch, neither parallel connection in series branch. Transmission 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 transmission 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.

Quit the program.

Toggle visualization of impedance (Z) circle

Toggle visualization of admittance (Y) circle

Toggle visualization of reflection coefficient (RHO) circle

Toggle visualization of constant Q circle

Set start point: can be entered as impedance, admittance or reflection coefficient

Ctrl+0 (zero)
Set characteristic impedance z0

Set frequency (f0)

Add a resistor

Add an inductor

Add a capacitor

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 element

Delete the currently highligthed element

Shift+Curs Up
Highlight previous element

Shift+Curs Down
Highlight next element

Shift+Curs Left
Decrease the first field of currently highlighted element

Shift+Curs Rigth
Increase the first field of currently highlighted element

Mod+Curs Left
Decrease the second field of currently highlighted element

Mod+Curs Rigth
Increase the second field of currently highlighted element

Mod+Curs Up
Increase the tuning step

Mod+Curs Down
Decrease the tuning step

Write network file in spice format

Restart for a new calculation with a clean chart

Change first field of currently highlighted element

Change second field of currently highlighted element

Start autotune procedure (tune goal is characteristic impedance)

Toggle lock flag in autotune procedure

Write an EPS or PS file

Load a previously saved .B gsmc job

Save a .B gsmc job

Increase frequency

Decrease frequency

Check online keystroke for an updated list of commands.


For operation that requires data entry a widget is created; it is composed by an text entry field and optionaly some radio button to chose the format of data entry.

The format for data entry can be a single number, or a more complex string. First of all every number can be supplyed using standar prefix for unit of measure i.e.:

(atto) 1e-18
(femto) 1e-15
(pico) 1e-12
(nano) 1e-9
(micro) 1e-6
(milli) 1e-3
(kilo) 1e3
(mega) 1e6
(giga) 1e9
(tera) 1e12
Note that for spice output format mega is written as MEG instead of M, that will be otherwise recognized by spice as milli. Unit of measure are not allowed to be in the text entered.

Where a complex number is required both the rectangular and the polar format are recognized: the former can be supplied as '12.3-j456m' while the latter can be supplied as '34.2 145' meaning a modulus of 34.2 with a phase of 145 degrees. If the complex number is purely immaginary only something like 'j82' can be supplied, while if immaginary part is zero a single number can be given. Immaginary operator can be issued as 'i' as well as 'j'.


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...


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 '800n', be sure that "Series" radio button is selected before pressing enter. TAB and Shift+TAB is useful 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 546nH.

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.

Transmission 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 length 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 retrieved 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 allows one 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 protected][email protected]