[sdiy] Analysing 4-stage RC filters without the hot air

[Tom Wiltshire]

Hi Neil,

Yes, it was me. I was struggling with using impedances to analyse these things. It's ok for simple cases, but I couldn't get more complicated things to come out at all. This wasn't due to tedium - I was using the computer to do the sums. One thing I never got entirely clear from the discussion on the list was whether the impedance method actually *works* in more complex cases (in which case I had errors somewhere) or whether I was barking up the wrong tree (in which case my error was in the choice of method). Everyone just said "Oh, don't do it like that!". I got the right answer for a single pole RC, but couldn't make it work for two-pole or three-pole cases. 

The ABCD matrices look much more friendly! And similarly amenable to running the working on a computer. That's a very readable page and leads me through the steps needed to get from 0 to dangerous in a nice simple fashion. Nice work, and thank you.

I've got a question, though. Inevitably!

How would you go about doing this "in reverse"? E.g. Say I wanted a 2 stage passive filter with an attenuation of 10dB and a cutoff of 1KHz. What would I need to do to get back to some reasonable values? Still, if I can do it on a computer, exhaustively testing potential candidates is a realistic choice.

Thanks,
Tom


On 21 Mar 2016, at 21:07, Neil Johnson <neil.johnson71 at gmail.com> wrote:

> Hi all,
> 
> Some of you may remember a thread a little while back about a 4-stage
> passive RC filter (I think it was Tom who asked the original
> question), and ABCD matrices came up in the conversation.  I've
> written up a page on the theory behind them and how they can be used
> to analyse and design passive circuits:
> 
> http://www.milton.arachsys.com/nj71/index.php?menu=2&submenu=2&subsubmenu=14
> 
> As a worked example I used a 4-stage RC filter from Electronotes where
> I show that actually it's not that difficult at all to analyse the
> unbuffered 4-stage RC filter contrary to Bernie's protestations.
> 
> Comments welcome!
> 
> Neil
> --
> http://www.njohnson.co.uk
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