Recently on the qrp-l mailing list, there was a thread on circuit-simulation software appropriate for amateur radio use. George, WD0AKZ, replied with two URLs:
Scott, KB0KFX, also chimed in:
I’d recommend checking out LTspice for the free route. It is widely used by engineers in industry and also popular in the hobbiest community. The documentation is decent and doing a google search you should be able to find many ham radio related examples. The LTSpice Yahoo Group is also a huge repository of examples and howtos. Use their file index system in the Files section of the Yahoo group to search and download examples. I’ve evaluated many of the spice simulators for my work a few years ago and this is just about as good as the rest for linear circuit simulation. You can easily include other vendor models besides the included LT ic’s. (see Yahoogroup examples)
Random Notes:
I attended a TI Motor Controls seminar this past year that talked about complex motor control schemes and DSP algorithm development. The TI app engineer had all of his example simulations in LTspice….go figure.Once you’ve used it, I can build up qrp circuits like a multistage sallen key op amp filter in 10-15 minutes. (TI has some good free software for designing active filters). A few weeks ago I was playing around with a sallen key filter for something to add to the Rockmite and I wanted to get a better idea what it would actually sound like. I used a PC mic to record various wav files from my HF radio speaker with the radio bandwidth at max (~3.5kHz). It had multiple CW signals in the passband. LTspice allows the use of a wav file to be linked to a voltage source. This voltage source was fed into the sallen key filter with the output writing to another wav file. The post processed wav file was filtered by the op amp circuit and could easily hear the ringing because I was too aggressive of using a narrow bandwidth. I should have an online repository to post such experiments to share with the group, but I don’t at this time.
So, there you have it. If you’re interested at all in doing some circuit simulation, that should keep you busy for a while.
LTSpice IV is useful for designing ham radio circuits but you have to put some effort into understanding what you circuit is actually doing at high frequencies. The base diffusion capacitance of a BJT transistor is a lot higher than many people think meaning the input impedance is really low at high frequencies.