This is a strange mixture of a VCO and a switched capacitor filter based on the filter presented by Jan Hall in Electronotes.

This filter is based on the original idea by Jan Hall, as presented in Electronotes EN92 P14-15, which was in turn based on an idea in a 1974 issue of Electronic Design on a Biquart (twice the order) filter. A VCO drives a pair of analog switches, switching two banks of eight capacitors across op-amps (effectively creating multiple integrators), and thus a multi-passband or comb filter is created at f0, f1, f2 etc.

The VCO section has been re-vamped, based on the well known Electronotes ENS-76 VCO Option 1 as used in the ASM-1, Modulus, etc. This core was an obvious choice due to the amount of information, lists of suitable substitutes and so on that are available on the web. In this case, a cut-down version is used as its purpose was simply to drive the analog switches.

The VCO is connected via a signal processor to the input of a binary counter. The first three divisions are used to drive the address lines of the analog switches in the filter.

The remaining divider outputs are also available at the [FEEDBACK] switch which allows them to be fed back into the input of the filter itself, thus giving the filter an excitation signal that is perfectly in sync with its operating frequency. When configured like this, the filter becomes a very unusual oscillator giving quite complex, waveforms or even simple waveform sequences (ringing bursts).

An external [EXCITE(X)] signal can alternately be used to 'excite' the filter (enabled by selecting 'X' on the [FEEDBACK'] switch) although in this instance, the excitation can be out of 'sync' with the driver-VCO.

Two controls are provided within the structure of the filter - damping and bandwidth. Not all combinations of these two are actually valid, some resulting in silence, but none the less, quite an array of variations is possible.

If you are after something different, this fits the bill. After all, how many filters have a sync input?

Build Guide