Depending on the position of the switch S1 to input and output chains of the filter and oscillatory circuit formed by inductor L1 and capacitor C3 connect either a quartz crystal resonators B1, B4, B5, B8, or resonators B2, B3, B6, B7. The switching elements are diodes V1 - V8. Thus, when the switch S1 shown in the diagram, the DC voltage + 9...12 V is supplied through resistors R2, R6, R7 and R10 respectively to the anodes of the diodes V1, V4, V5 and V8. These diodes are open. The total current of these diodes generates a resistor R4, the voltage that covers the diodes V2, V3, V6 and V7 (their cathodes are connected through resistors R3, R5, R8, R9 and the switch contacts S1 to GND).
Fig.1
The inductance of the coil L1 (she has a branch of the middle) and the capacitance of the capacitor C3 depend on the used quartz resonators, the resonant frequency of the resonant circuit formed by these elements, should be approximately equal to the middle frequency of the bandwidth of both filters. On the final response of the filter is influenced by the resistors R1 and R11. Should be selected when setting up, observing the condition R1=R11.
The frequency of the series resonance of the quartz B1 should be the same with quartz B5, and B4 - quartz B8 (for the second filter, respectively, B2 - B6, and B3 - B7). The frequency spacing between these pairs of resonators depends on the required bandwidth filters. It can be found in Fig. 2, which shows the dependence of the bandwidth of the filter dF1 from the frequency distribution dF2 consecutive resonances quartz, forming the filter.
Fig.2
As switching diodes, you can use any modern high-frequency silicon or germanium diodes, for example. KDA. HD etc.
Literature
CQ (USA), 1978. No. 7
Publication: N. Bolshakov, rf.atnn.ru