I came across that SVF filter at that site a while ago and (after of course translating it to VHDL) I promptly expanded the test jig with more input waveforms and implemented all filter type outputs. So as inputs I can select noise, sqware, sine and sawtooth and for outputs I can select lowpass, bandpass, highpass and notch (band stop).
I dug up the test code and made a
to show what it can do. The parameters of the filter are, fsample=800Khz, Q=1 and you can clearly see that even at Q=1 it still amplifies the signal at the filter's cut off frequency. Due to the internal number representation use I cannot lower the Q any more and of course increasing the Q will cause even more amplification to the point of overdriving the output (causing math overflows).