[sdiy] Measuring filter response

[Harry Bissell]

Justin:

The post describes a way to visualize the frequency response of an audio system (what you want)
but to be practical you need a storage oscilloscope for it to work. 

I'd start with a sine wave input, and measure the amplitude in the passband (well below cutoff), then
adjust the sine wave input until the filter output falls the -3dB. This frequency is the cutoff frequency
by definition.  If you have a sine wave generator you could probably know with some precision what frequency that was.

You could do the same at the -12dB point, then compare the data for both filters.

Not a lot of fun. What are you trying to find out... filter tracking of each other ?

H^) harry


----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 15 Aug 2011 14:22:26 -0400 (EDT)
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.

On my scope I'm seeing a dot move up and down with increasing speed (as the frequency of the sine wave input into the filter is increased by the ramp wave driving the VCOs CV input?). The dot eventually moves so fast that it appears as a solid vertical line (highest frequency?). The dot/line is also moving left to right (again, I'm assuming this is matching the rise of the ramp V PP). If I lower the cutoff frequency on the filter the vertical pitch of the dot decreases. Since I'm currently measuring the low pass output and the input is a sine wave - I'm assuming this is the input being attenuated?

Problem is, I don't fully understand what I'm seeing or what I'm supposed to be seeing or reading/measuring or whether this test is even related to what I'm after. Part of the problem might be that my Owon scope turns off a lot of the measuring facilities in XY mode.

I did also try using white noise as the filter input and viewing the output on a spectral analysis plug-in - but trying to 'measure' anything on this was pretty hit and miss.

Any input on this - what should I be measuring, what am I supposed to be looking at with the scope config described above?

Thanks,

Justin





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-- 
Harry Bissell & Nora Abdullah 4eva