Universal VCF - voltage controlled bandpass filter

The term voltage controlled filter (VCF) frequently crops up in connection with synthesisers. As its name suggests, a VCF is a filter that is controlled and adjusted by applying different voltages. This particular circuit consists of a voltage controlled audio bandpass filter with a variable IF and bandwidth. At the heart of the circuit there is an active bandpass filter around A2. 33 n capacitors are connected in parallel to the frequency- determining 1 n capacitors by way of ES3 and ES4. The electronic switches are controlled by means of a high frequency signal having a variable duty cycle. When an electronic switch and a capacitor are connected in series, they have the same average duty cycle as a variable capacitor. This enables the intermediate frequency (IF) range of the filter to be adjusted. Similarly, ES2 affects the gain of A2 and there- fore the bandwidth, or rather the Q factor. Unfortunately, a reduction in bandwidth in this type of filter automatically leads to a rise in