Homemade Tunable Crystal Filter

It takes only four crystals to build a tunable crystal filter.

Rolf Heine, DL6ZB

This quartz crystal filter is tuned by a variable voltage. The impedance is between approx. 40 and 150 ohms, with bandwidths from 3 kHz down to 300 Hz. A steeper roll-off and higher stopband attenuation can be achieved by cascading these crystal filters, at the expense of higher passband attenuation.

Due to parasitic effects, in particular parasitic capacitances in the structure of the crystals or the RF transformer, sharp secondary resonances at about −30 dB may occur a few hundred kilohertz below or above the fundamental frequency. Cascading two or more filters almost completely eliminates this effect.

I have found BZX85C15 Zener diodes to be a good substitute for more expensive varactor diodes. Although this filter works at any fundamental crystal frequency, I recommend center frequencies of around 8 to 12 MHz for best performance.