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

[Neil Johnson]

Hi Tom,

> 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.

Thanks.  I'll be adding a section on the twin-T filter as well to
illustrate using Y matrices to handle parallel cases.

> 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.

Well, firstly it would help to add more to your filter specification -
how much ripple in the passband and in the stop band, whether
preserving phase is important or not, and so on.  Once you have that
info then I think your choices fall into two options:

1) the easy way is to grab a copy of a standard filter tables
handbook, look up the filter type you need, pull out the coefficients,
and frequency and impedance scale to your specific needs.

2) the harder way is to go back to basic theory, slog through the
maths (in essence repeating what the authors of the filter tables
books did) to arrive at the same, or near enough the same, answer.

I would suggest getting hold of a good filter handbook.  I have
various ones, including Williams/Taylor "Electronic Filter Design
Handbook" and Zverev's "Handbook of Filter Synthesis".  There are
other filter design books out there, but for just synthesizing
something starting from a spec you just need the tables.

Cheers,
Neil
--
http://www.njohnson.co.uk