(from TM 11-866)
14. CRYSTAL FILTER
a. The Quartz Crystal Filter (T1) couples the 1st detector (V3) to the 1st IF amplifier (V5). Its selectivity can be varied in definite steps by the CRYSTAL SELECTIVITY switch (SW7) controlled from the front panel by knob and pointer E7. In addition, its selectivity characteristics can be greatly sharpened on one side or the other by adjusting the PHASING condenser (C32), which is controlled by knob E8.
b. When the CRYSTAL SELECTIVITY switch is set at OFF the quartz crystal is short-circuited and signal voltages present in the secondary of the 1st detector plate coil (L26) are impressed directly on the control grid of the 1st IF amplifier tube (V5). At any other setting (1 to 5) the quartz crystal is in use and acts as an extremely high "Q", high impedance, series tuned circuit interposed between the secondary of plate coil (L26) and the 1st IF grid circuit (L27, C33), which constitutes the load into which the crystal works. Selectivity is varied by altering the impedance of this parallel tuned circuit (L27, C33), which is accomplished by adding resistance (R42, R43, R44, R45) in series with coil L27 and capacitor C33. As this series resistance is increased (reducing the parallel impedance of circuit L27, C33) the overall selectivity of the filter is also increased.
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