[sdiy] Measuring filter response

[Dave Brown]

I forgot I had these on my synth tips page.  I have XY plots taken from of a
Korg MS-20 filter.  I did this after a repair as a quick verification of
functionality.  I would hate to actually try and take a measurement from
this technique, but it is good to see an overall response.  In the examples
on my page, you can see a DC shift at the extreme cutoff of the LPF.  You
can do this reasonably well without XY mode by just displaying a dual trace
of the low frequency ramp sweeping a VCO and the output of the filter as the
second trace.  You will see the overall envelope of the filter response.
For an accurate measurement I would just monitor the VCO frequency and the
amplitude of the filter output at the various frequencies of interest. -
Dave

http://modularsynthesis.com/modules/tips/tips.htm

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Justin Owen
Sent: Monday, August 15, 2011 11:22 AM
To: SDIY List
Subject: [sdiy] Measuring filter response

Hello,

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.

I was reading this post from 2007:
http://search.retrosynth.com/synth-diy/search/lookit.cgi?-v0707.518 on using
the XY mode of a scope to measure filter response.

Justin