I am experimenting with VCFs for an analogue music synthesiser. Initially I tried a 3-stage low-pass filter based on the sinewave-generator circuit in the LM13700 datasheet but having removed the oscillator, as per schematic below:
I find that its effect on a squarewave, as the filter frequency is swept, is not symmetrical. A series of scope traces shows the effect of the sweep:
If the low-pass filter is meant to attenuate higher harmonics, I would expect to see the square soften to a sine whilst retaining its symmetry. Using a Max293 active switched-capacitor filter does give a more symmetrical sweep (ignore the apparent clipping, it's due to the input running to the rails):
Question: Can anyone explain in simple terms why these filters have different effects on the waveshape symmetry (I assume it's something to do with phase shift in the analogue circuit) and whether the asymmetry can be avoided by using a different analogue design rather than having to use an active filter (with its associated downside of the clock frequency breaking through)?