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Free Simulator

PostPosted: Thu Aug 19, 2004 10:49 pm
by N9NEO
I fumbled through a brief presentation at Boxboro hamvention last week. Steve was kind enough to allow me the time to make a presentation. The topic was simulating circuits with SwitchercadIII.

Get it here:

Switchercad is a very fast spice simulator that is offered FREE from Linear Technology. I've never had a problem with the thing not converging - unlike the high end commercial package that I use at work. It has a very nice schematic capture module with an equally good waveform viewing package. You snap on a node with a scope probe for current or voltage waveforms. You can perform basic math functions on waveforms as well. Spectrum analyser output is easy. This can be very useful for designing output filters for Class E type rf decks. AC signal sweep mode is very useful for designing filters and things like this as well.

I designed a mic preamp in about an hour with switchercadIII, complete with Bode plots of frequency response. With a little practice you could probably get a complete Class E rig running in a few minutes. I have a complete modulator and rf deck simulated and it runs very fast.

The parts library is as good as what you would find in a commercial offering. Linear Technology makes some of the best analog parts available. Fast comparators, accurate rail to rail op-amps, etc. Their motivation for producing this freeware is to sell parts, and this strategy has not failed them. There is a very large community out there using Switchercad. I urge you to give this package a spin. Don't be intimidated by it's capabilities, or think it can't be too good if it's free. It's easy to learn, and powerful as well. I will share some of my files, and there is also a Yahoo web site dedicated to this software package.


PostPosted: Fri Aug 20, 2004 8:01 pm
by ke1gf
Thanks for all your help Bob.

I've taken my R&D work for my new driver circut and started to reconstruct it in SWCADIII, looks promising.

Looks like I'm going to need a BIG cup of coffee to get familiar with it.


PostPosted: Fri Aug 20, 2004 11:24 pm
by ke1gf
Woah, good stuff!

I climbed the curve tonight, I've got the digital and MOSFET side of my driver modeled and working.

It's pretty easy to use regular spice models with it, I've got a few vendor supplied ckt's working...

Bob, you're a Gentleman and a Scholar.

All the Best.

PostPosted: Sat Aug 21, 2004 1:25 am
by ke1gf
Well I took my quazi totem-pole driver and hooked it up to a L-C series outpoot and made a real simple 1W Carrier Piss-Weaker, need a modulator though.


This software is a lot of fun, I have OrCAD PSPICE but this package seems alot more geared up for what we're doing.


Good on getting it up!

PostPosted: Sat Aug 21, 2004 11:48 am
by N9NEO
FB on getting it up and running. Rather easy when you get down to it. And yes, not a very steep curve to get up. By far the easiest package I've used.

One thing not so obvious - for a transformer use 2 inductors. Use spice directive pull down menu to link the two inductors. If you find the transformer wired backwards just flip winding around. (Not sure if coupling constant can be negative value)

K1 L1 L2 1
K2 L3 L4 0.99
Kn Lx Ly 1

Where last term denotes the coupling factor. depends on geometry, but .99 to 1 is good call. Can just as easy use 1 and use a seperate inductor in series with transformer to model leakage reactance.

Also for spectrum analyzer output just a pull down in waveform display. Make sure you have a stable waveform before doing fft. Not sure if you have to fin an integer number of cycles into window though.

Now if we have a place to stash files on line that would be good for people to share class E type files. I don't think the Yahoo site dealing with switchercad likes to store files other than a temporary basis. Maybe another Yahoo site for RF type stuff.


PostPosted: Sat Aug 21, 2004 4:12 pm
by ke1gf
Bob, being the newbie that I am is there anyway that I can have SWCAD III tune up the rig, lets say tweak a component for maximum current through a resistor, my values are close but you know how it is?

I tried keywords in the help system like "solve, optimize, optimum, maxima, tweak"

your transformer info helped quite a bit, I'm getting a nice healthy 10w of of my D rig now.


PostPosted: Sun Aug 22, 2004 8:37 am
by N9NEO

I don't think Switchercad will tune for you. You have to do that
for yourself. It has no idea what an optimum circuit is. There
is some type of efficiency report that it will output, but being a
newbie myself, I haven't gotton around to investigate that
aspect of package.

The package closely follows GIGO rules. Garbage in Garbage

As an example, if you model a transformer with a coupling
of 1, an ideal transformer, you're not going to do as well in
practice as simulation would predict. Normally for rf stuff the
leakage reactance will limit the practical turns ratio you can
successfully acheive. The simulation will predict more output
voltage as more secondary turns are added. In practice, the
leakage inductance will go up fast (N^2) so that the output
remains constant, or may even go down with more turns.

Another example would be parasitic inductances in mosfet
leads. You may end up with ringing on actual waveforms that
you may not have seen in simulator.

A good tool to have in your box, but not as useful as a wheel.


PostPosted: Sun Aug 22, 2004 12:16 pm
by ke1gf
I guess from what I've been reading online there is a way to do it. With a High-Level scripting language called TCL? write my own optimization loop and call the spice programs from that...

sounds like a pain in the butt to me.

-Bill 'GF