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Junk Science

There's an awful lot of bad engineering about these days. How do you know if those circuits you pick up off the internet will really work? The bad news for most of us is that we don't know. And unless we build them we can't find out. Even then, they may not be the best solution to the problem. Maybe a simple change - based on sound engineering - would have made the circuit function better. So, how can you test those gems of wisdom from the net?

Let's consider two designs for filters. Both purport to be lowpass filters, both are on the web - but only one works (but see comments below about that one too!).

Figure 1

Figure 2

I guess that most people (including me) could not tell how these circuits perform. How to check in less than half an hour? Enter PSPICE.

Comments received

I just noticed that both op amps in the first
circuit have their feedback to the positive inputs. So they should not work
as linear amplifiers at all. This is probably a drawing error, caused by
the op amp model provided by Pspice, which has the + input on top. Analog
circuit designers usually draw op amps with the - lead on top. The lower
circuit has feedback to both + and - inputs, but it's correct. Also, the
uA741 op amp is very old, and was designed for 15 volt supplies. With 5
volt supplies, the output voltage swing might be only a volt or two.

I've been using Pspice at work since 1988. I love it. I used to think that
I knew everything about analog circuits, but Pspice taught me a lot of new
stuff. It's almost never wrong.

----- Original Message -----

> On Sat, 13 Nov 2004 19:38:10 -0800, Gene Mirro <[email protected]>
> wrote:
>
> > About your junk science page: the first circuit will roll off below
> > 1591Hz because of C1. Also, the first circuit has a capacitive load to
> > ground at the output of U1, which will probably create instability. The
> > second circuit has real serious problems. There is no DC return path
> > from pin 3 of U2, so it is probably not functional. The input DC bias
> > current needs a path. The problem is that if you add a resistor to
> > ground, you will then get a low-frequency rolloff due to C5. Both of
> > these circuits are bad.
>

PSPICE Revealed

PSPICE is a software circuit simulator that allows complex systems to be accurately simulated on your PC. It includes a schematics drawing package, circuit simulator and PCB layout - all in one integrated software suite. Now I have known about PSPICE for ages but have never bothered with it because it was sure to be:

Boy, was I wrong!

The other day, I went to the PSPICE site and, having recently got unmetered internet access, I downloaded the student version of the software - which is free. OK, I was sure that it would be extensively knobbled so that it was pretty useless but as I only wanted to simulate a simple matching network, I thought that it might do the trick. Hours later (it's many megabytes long) I installed the software. You can go straight to the download page here (27Mb). Not a student? - You're learning aren't you?

Expecting a long learning process I started off looking at it. I hadn't downloaded the manual so it was an intuitive look. But hey, within half an hour I had my filter drawn, and less that quarter of an hour later I had the simulation done. This is really great software. I can do complex stuff without even looking at the manual. But it's knobbled right? WRONG. The student version does it all. It's limitation is a limited parts list but that limited list is huge. I'm sure that it will meet most people's needs for ham radio use. Don't feel guilty either about using this powerful software for free. In the About Box for the PSPICE circuit simulator it says "copying of this program is welcomed and encouraged".

Back to those filters

After my initial success with the simple four component passive filter, I decided to try active filters. PSPICE makes this just soooo easy.

Here are our two filters again, this time showing how they perform.

Both said they were lowpass filers. Which is which? Download PSPICE and find out for yourself!

Caveat

Like most software, PSPICE relies on the designer to know what he/she is doing. PSPICE will require some degree-level knowledge of electronics to get the best out of it. Try it and see.

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