PSpice and the Ham -
or, Serenade for the Serious

Do a lot of your breadboard fiddling in the virtual realm...
Simulation Software It's all over the place in the technological realm - complete communications systems from concept to PC board can be tweaked pretty far just on the computer screen.

ARRL recognized this a few years ago, and persuaded a major vendor of RF simulation software (Compact Software) to make a "lite" version of an industrial simulator, and release it as "ARRL Radio Designer". Trouble is, it was limited to the cumbersome text-entry style known as a "netlist". The scoop on this is at . A series of QST articles ran on the use of this software, all available at that site, with netlist files included. (Those files can be opened, with some modifications needed, in Serenade referred to below.)

Fortunately, one of its progeny is available free!  Compact Software was acquired by Ansoft, and now marketed the Serenade package. Its free student version was Serenade SV, which I played with a bit. It seems to have been superseded by - give it a try.

But wait, there's MORE out there for free! SPICE (in various flavors) is a standard of the industry for general simulation. Lots of older versions of SPICE are floating around on the net. I even have an old Mac version that doesn't quite adhere to the Mac interface, but worked for me. Orcad,, seems to provide the best free versions. I used Orcad 9.2 Lite Edition on a free CD from them, and now OrCad 10 is available via a very large download, ...

These are descendants of the MicroSim Design Lab suite, which was also available in a free lite version. This is what I've dabbled with over about 4 years. They're limited mostly  in design size you can save. You can still do plenty within the limits, of course. 

Just to round out the picture, I'll mention the other major simulator for the education market, Electronics Workbench, from I have an earlier student version of this software, which cost about $75 as part of a certification training package. It is a lot easier to use, and runs more like a breadboard and test bench, with virtual test instruments like an oscilloscope and volt/amp meters, and switches that change state when you hit the space bar. The newest version, Multisim 2001, has a free demo, but with a 25-part limit and no save capabilities. Student and personal editions aren't too expensive, as quality software goes. It's surely worth a try if you want to spend some money. 

An OrCad  Schematic To start out, I modelled the keying waveforms of one of my RF driver stages, and it came out pretty close to what I have actually achieved over the years. Since I haven't downloaded all the parts models yet, I'm temporarily using another RF transistor in the model than what I'm actually using (a 2N5109). 

Now this schematic for modelling has a few things added you wouldn't see in the construction schematic - the DC and AC voltage sources, and the source and load resistors, for example. Most notably, the little keying pulse generator inside the dashed box simulates a keydown-to-ground output from a key or keyer for a bunch of fast dits. (If there's a better way to do this, please e-mail me, somebody!)

Once you draw that schematic in Capture, you set up simulation profiles. The "transient" analysis is basically an oscilloscope display - time vs. amplitude. This one is a 100 millisecond run - and it takes a while, because you're viewing the overall RF pulse shape of a keyed CW waveform. To save some time, I made the signal source only 100 kHz - otherwise it will take forever. As it is, the actual output file is 88 megs for this measly 100 milliseconds of simulated circuit operation, and took about 5 minutes to run on my P3-600. Then you place the "markers", the little colored scope probes you see above. And voilà:

 PSpice A/D Output from the Above Here's the scope-like output of the "probe" function of PSpice A/D Lite. The yellow trace is the keying generator output; the red trace is the shaped DC voltage to run the amp; and the green waveform is the shaped RF output of the amplifier. I went for a fast but clickless rise and fall time for my QRP CW work - about 1 ms rise, and 2 ms fall. You could of course shape this according to taste by the values of C5, R7, and R8. 

This is after-the-fact work for me - I got this by experimentation about 3 years ago! But it's fun to see the correlation. 

Things I've modelled for fun:
Tuned filter networks, like input bandpass filters for receivers, are fun to tweak on-screen for various combinations of bandwidth and frequency. I've always looked for good compromises in my fitlers when tuned over a band of frequencies. 

One interesting discovery I made was in the behavior of a series-tuned circuit operating at low impedance with a step-down transformer (50 ohms to about 5 ohms). Its behavior is yet to be verified with real components, but this circuit:

yields this result, at least in PSpice...

Over the tuning range, the bandwidth of the filter (in terms of MHz/kHz) stays remarkably uniform. Transformers are 20:1 impedance range (about 4:1 turns ratio); L is 2uH; and in this example C tunes from 20 to 320 pF. We'll see how this works in real life...

e-mail to k0jd at seboldt dot net (note spamfoil format - type in regular address format ;) )
Milwaukee, WI

This page last updated on Dec. 19, 2000