20 m variable crystal oscillator

How it works

A variable crystal oscillator pulls a crystal away from its nominal frequency using a reactive network. CV1 adjusts the frequency and the buffer isolates the oscillator from its load. L1 is especially important to the achievable tuning behavior.

The archive reports about 0.2 V peak-to-peak output, 36 kHz tuning with one crystal, and 250 kHz with two. These figures depend strongly on the actual crystals and have not been independently reproduced.

On the bench

Begin with one crystal and measure frequency drift and tuning range. Add the second only after establishing a stable baseline, and use an additional buffer where the load would otherwise disturb the oscillator.

Resources & provenance

Rewritten from the original circuits/rfr/hg65q0p_rfr015.php page. Component values and historical performance claims have not been independently bench-tested. The circuit figures have had their watermarks removed.