filter waveform question

[Martin Czech]

> I have built a diode ladder filter and have a question about the output
> waveforms I am getting.  I have posted some oscilloscope photos at:
> 
> http://www.evenfall.com/diy/dladder.html
> 
> I am wondering if anyone might know what is causing a "jump" or
> discontinuity near the peak of the waveform when the resonance control
> is turned up.  I have spent hours tinkering with the circuit and can't
> figure out what is wrong.  Sigh.
> 
First impression: nothing wrong.

The problem is: if you use an input wave with harmonics (saw) and you
don't really know your filter transfer function for magnitude and
phase, you can't see much out of such a screen shoot.  Because every
single harmonic will get some (unknown) phase shift the output waveform
may have nothing to do with the incomming.  E.g.: you know that a saw
is combined of partials with magnitudes like 1/1, 1/2, 1/3, 1/4 ...
etc.  If you have got enough of them you can perfectly synthesize the
saw sound. The phase of each partial can have random value, it doesn't
matter in this case. If you look at the waveform, it will most probably
not look like a saw at all, the (unimportant) phase information gives
you a lot of freedom to make a lot of waveshapes that sound the same
(for the continous case).

Therefore it is a better idea to test a  (unknown) linear system with a
sine wave. If the amplitude is not to large, a nice sine wave should
appear at the output. The output wave can vary in magnitude and phase,
you can measure this with your scope for different input frequencys and
thus get an impression of the transfer function.

If your filter can't deal with a sine wave as described , then there's
something wrong, for shure.

m.c.

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