Sadly doesn't work on EC diodes, usually you would use varactor diodes specifically designed for this, but normal diodes do it aswell to some extent
How it works: The diode can be seen as a capacitor, with two plates (P/N layers) and the depletion zone as dielectric. If you apply reverse voltage to the diode, the depletion zone grows, further seperating the "plates" and therefore reducing the junction capacitance. The two 22n caps are just for DC blocking.
This concept has been used a lot in radio receivers from the early 70s to the 1980s as tuning condensers.
It has several advantages, a mechanical tuning condenser is expensive, big, and not standardized. Cheaper, smaller ones with plastic layers between the plates like used in the Tivoli model one have been proven unreliable.
This concept here can be implemented with cheap standard parts though and using small multiple pots allows you to switch inbetween them and have adjustable station presets without all the digital complexity.
