Tektronix 1L30 Instruction Manual page 35

.Circuit Description— Type 1L30
Fig. 3*7. Crystal variable resolution filter.
in series between the input and the parallel resonant cir­
cuit L508-C508. Bandwidth or resolution of the circuit is
dependent on the characteristic response of the crystal
at its series resonant frequency and the Q of the parallel
resonant circuit L508-C508.
Fig. 3-7 illustrates the impedance response versus fre­
quency curve of a quartz crystal. Capacitor C504 neutral­
izes the. stray shunt capacitance around the crystal so the
response of the crystal is equivalent to a series tuned cir­
cuit with a very narrow bandpass1; see Fig. 3-8.
' (Ref: F. langford-Sm'ith RAC Radiotron Designer's Handbook; fourth
edition.)
3 -8
Fig. 3-8. Crystal filter, equivalent circuit and impedance response
curves.
The bandwidth of the filter network is a function of the
crystal output load, which is primarily the parallel resonant
circuit; therefore, bandwidth
Q for the resonant circuit. The Q of the output load cir­
cuit for the crystal is varied by changing the bias of diode
D506, which changes the shunt loading across the parallel-
tuned circuit.
As the forward bias o f D506 is increased, the Q of the
parallel resonant circuit decreases and the response charac­
teristic of the crystal becomes the dominant factor in de­
termining the bandwidth of the filter network. The crystal
becomes a function of the
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