User's Manual

9
Output signal from phase comparator is filtered through a low pass filter and passed
to the VCO to control oscillator frequency.
2) VCO
The operating frequency is generated by Q352 in transmit mode and by Q350 in
receive mode. Operating frequency generates a control voltage by phase
comparator to control varactor diodes so that the oscillator frequency is the same as
the MCU preset frequency (D350, D352, D354 and D355 in transmit mode and D351,
D353, D356 and D357 in receive mode). T/R pin is set high level in receive mode
and low in transmit mode. The output from Q352 and Q350 is amplified by Q354 and
sent to buffer amplifier.
3) Unlock Detector
An unlock condition appears if low level appears at LOCK pin of IC400.
Transmission is forbidden if this condition is detected by microprocessor.
3. Receiver
The receiver utilizes double conversion superheterodyne.
C
ANT
ANT
SW
BPF Q201
RF AMP
BPF
MCF
XF201
IF AMP
Q222
MIXER
IC218
CF201,202
IF,MIX,DET
IC204
AF AMP
IC720 SP
1st Local OSC
(PLL)
2nd Local OSC
IF AMP
Q216
Fig.3 Receiver Section Configuration
1) Front-end Amplifier
The signal from antenna is amplified at RF amplifier (Q201) after passing
through a transmit/receive switch circuit and a band pass filter. The amplified
signal is filtered through a band pass filter to remove unwanted signals before
it passes the first mixer.
2) First Mixer
The signal from RF amplifier is mixed with the first local oscillator signal from PLL
frequency synthesizer circuit at the first mixer (IC218) to create a 44.85MHz first IF
signal. The first IF signal is then fed through a crystal filter (XF201) to further remove
spurious signals from adjacent channel.
3) IF Amplifier
The first IF signal is amplified by Q216 before passing through crystal filter and by
Q222 after crystal filter and then enters IF processing chip IC204. The signal is mixed
with the second local oscillator signal again in IC204 to create a 455KHz second IF
signal. The second IF signal then passes through a 455KHz ceramic filter (wideband:
CF201/narrowband: CF202) to eliminate unwanted signals before it is amplified and
detected in IC204.
4) Narrowband/Wideband Switch Circuit
Turn on ceramic filter CF201 (wideband)/CF202 (narrowband) to set each channel as