Simple crystal filter.

Igor Chernikov, UA3DJY, email c_igor at inbox.ru

February 08^th, 2011, updated on March 31st, 2011

Crystals should be designed to work with serial resonance with no embedded capacitor inside.

Filter parameters: center frequency 7010.75kHz, bandwidth 550Hz, passband losses 7dB, stopband losses close to 60dB.

Bandwidth and passband losses depend on dynamic resistance of the crystals. Specified parameters are achieved using poor crystals with different resistance: one was 55 Ohm other 250 Ohm.

Capacitor C1 and quality factor of crystals affect on value of the bandwidth and losses in the stopband. Capacitor C2 is used to tune frequency of the second crystal and affects on bandwidth value and frequency response characteristics ripple in the passband.

L1 is used to compensate crystal holder capacitance, improves rejection in the stopband.

Bipolar transistor buffer is used to match output impedance of the filter to 50 Ohms, it lets to save filter rejection in stopband if filter impedance differs from 50 Ohms.

Switch, is not shown on the diagram, used to turn on/off the filter.

Graphs with the frequency response characteristics: