Johnson DM3474, Service Manual

Page: 30 / 64

CIRCUIT DESCRIPTION
4-6
Part No. 001-3474-002
4.2.3 VCO AND REFERENCE OSCILLATOR MODULATION
Both the VCO and reference oscillator (TCXO) are modulated in order to achieve the required frequency
response. If only the VCO was modulated, the phase detector in U801 would sense the frequency change and
increase or decrease the VCO control voltage to counteract the change (especially at the lower audio frequencies).
If only the reference oscillator frequency is modulated, the VCO frequency would not change fast enough
(especially at the higher audio frequencies). Modulating both VCO and reference oscillators produces a flat audio
response. Potentiometer R810 sets the VCO modulation sensitivity so that it is equal to the reference oscillator
modulation
sensitivity.
4.2.4 CASCODE AMPLIFIERS (Q851/Q852)
The output signal on the collector of Q850 is coupled by L861/C864 to buffer amplifier Q851/Q852. This is a
shared-bias amplifier which provides amplification and also isolation between the VCO and the stages which
follow. The signal is direct coupled from the collector of Q852 to the emitter of Q851. The resistors in this circuit
provide biasing and stabilization, and C865 and C866 are bypass capacitors.
4.2.5 AMPLIFIER (Q853)
Amplifier Q853 provides amplification and isolation between the VCO and receiver and transmitter. C868
provides matching between the amplifiers. Bias for Q853 is provided by R871, R872 and R874. Inductor L856 and
capacitor C873 provide impedance matching on the output.
4.2.6 VOLTAGE FILTER (Q832)
Q832 is a capacitance multiplier to provide filtering of the 4.6V supply to the VCO. R836 provides transistor
bias and C834 provides the capacitance that is multiplied. If a noise pulse or other voltage change appears on the
collector, the base voltage does not change significantly because of C834. Therefore, base current does not change
and transistor current remains constant. CR832 decreases the charge time of C834 when power is turned on. This
shortens the start-up time of the VCO. C841, C840 and C855 are RF decoupling capacitors.
4.2.7 VCO FREQUENCY SHIFT (Q831)
The VCO must be capable of producing frequencies from approximately 403-564.95 MHz to produce the
required receive injection and transmit frequencies. If this large of a shift was achieved by varying the VCO
control voltage, the VCO gain would be undesirably high. Therefore, capacitance is switched in and out of the tank
circuit to provide a coarse shift in frequency.
This switching is controlled by the T/R pin shift on J201, pin 4, Q831/Q834 and pin diode CR850. When a pin
diode is forward biased, it presents a vary low impedance to RF; and when it is reverse biased, it presents a very
high impedance. The capacitive leg is switched in when in transmit and out when in receive.