[sdiy] Measuring filter response

[Justin Owen]

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