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|  TM 11-5820-670-30
1-26. LOCAL OSCILLATOR MODULE A1500. (CONT)
TUNING CIRCUITS
The tuning circuits control the frequency of the oscillator. They contain resonant tank circuits with a
mechanical linkage from a variable inductor to the gear train which enables the radio set to tune to the
frequency set by the control unit automatically.
A dc error signal from the crs is applied to a varactor (variable capacitance diode) in the tuning
circuits to alter the tank capacitance, thereby changing the resonant frequency and operating
frequency of the oscillator. The dc error voltage from the crs keeps the oscillator running to within
3.5 kHz of the frequency selected on the tuning dials.
LOCAL OSCILLATOR Q1501
Q1501 is a pnp transistor designed into a modified colpitts configuration. Frequency tuning by the
tuning circuits enables the oscillator to operate within a range of 41.50 MHz through 64.45 MHz. The
oscillator applies the frequency to mixer module A1400.
For all frequency settings on the control unit in
band range, the local oscillator o crates 11.5
MHz above the selected frequency. For all frequency settings on the control unit in B band range,
the local oscillator operates 11.5 MHz below the selected frequency. For some examples, see the
following table.
RANGE
LOCAL OSC.
CHANNEL SELECTED
BAND
OUTPUT TO
BY TUNING DIALS
53-75.95 MHz
MIXER
30-52.95 MHz
SELECTED FREQUENCY
41.50 MHz
30.00 MHz
40.55 MHz
52.05 MHz
52.95 MHz
64.45 MHz
53.00 MHz
41.50 MHz
65.00 MHz
53.50 MHz
75.95 MHz
64.45 MHz
BUFFER AMPLIFIER Q1502
oscillator signal being applied to mixer V1401.
BUFFER AMPLIFIER Q1503
oscillator signal being applied to the crs. The Iocal oscillator applies a signal to the crs through buffer
amp Q1503 for the purpose of having a dc error voltage applied back to the local oscillator.
1-25
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