[sdiy] Measuring filter response

[Neil Johnson]

Hi Justin,

> I'd like to measure the cutoff frequency response of the two filters
> in my modular. i.e. - I'd like to see how close they get to -3db at
> the cutuff frequency and a -12dB per octave decrease across a range
> of frequencies. Both filters are 2 pole, 12dB SVFs.

How accurately do you want to measure to?  And how quickly do you want 
the result?

Several methods I've used in the past include:

1) Spectrum analyser.  Fastest, most expensive.

2) Decent multimeter that does dB scale (e.g. any decent Fluke) plus a 
decent sinewave gen.  Takes a little while, medium cost.

3) Decent sinewave gen, plus dual trace scope and a switched dB box, 
monitor the filter output on one trace, and the filter input through the 
dB box on the other, at each freq adjust the dB box so both traces are 
the same amplitude.  Slow, cheap.

4) Use a PC program that uses the soundcard as a low-cost spectrum 
analyser, such as Visual Analyser.  With a white noise source you can 
plot the filter function and then use a ruler to measure the slop. 
Cheapest of all but pretty quick.

Cheers,
Neil
-- 
www.njohnson.co.uk