Last year, we published a high-quality, variable-frequency audio notch filter. To complete a typical test lab setup for measuring audio equipment performance, this article introduces a high-performance audio frequency generator with continuously adjustable output frequency control. The key component in both these designs is something called a State Variable Filter.

A State Variable Filter is at the core of this design and consists of analog integrators together with a carefully controlled feedback coupling to create a band-pass filter with a very high quality (Q) factor. The level of performance achieved by this filter would be difficult to attain using alternative, more basic circuit designs. The band-pass filter is made to oscillate by positive feedback. By combining this with active amplitude control in the feedback loop, we can build a high-quality, low-distortion adjustable audio frequency generator.

The Block Diagram

The band-pass filter, shown in the block diagram of...