Service Manual

10
U202 generates a control voltage via the phase comparator to control varactors (D100 and D101 in
transmitting mode; D102 and D103 in receiving mode) to bring the oscillator frequency of VCO in
line with the preset frequency of MCU within a broader frequency range.
The switching tube Q652 switches between transmitting and receiving under the control of T/R. In
transmitting mode, T/R is set as low level and Q100 operates when Q653 becomes conductive. In
receiving mode, T/R is set as high level and Q101 operates when Q653 cuts off. Output from Q100
and Q101 is sent to the buffer amplifier for process after passing through amplifier Q102.
If PLL is unlocked, LD pin of U202 outputs low level. When this situation is detected by a
microprocessor, transmitting and receiving operations are suspended and an alert tone sounds.
RF Power Amplifying Circuit (Tx Section)
Block Diagram for RF Power Amplifying Circuit
R/TSW
L453
1u
RX
RQA0002
2
13
3 Amplifier
Q401
2SC4988
1
23
APC
LPF
2SK3475
2
13
Q400
2SC5108
1
23
Q104
2SC5108
1
23
MATCH
ANTENNA
Q402
0
RF PA
Q403
Figure 3
To obtain the required RF power, RF signals from VCO is amplified by driving amplifiers Q400 and
Q401 after passing through buffer Q104. The amplified RF signals enters driver Q402, which
performs power pre-amplification on the input signals to derive a certain power to drive the final
power amplifier. The final power amplifier Q403 performs power amplification again on the input RF
signals to derive the required power. Then the amplified RF signals pass through Tx-Rx switching
diode D401 followed by the LC low pass filter circuit (LPF). The signals are transmitted through the
antenna after ultraharmonics are filtered by the LPF.
Rx Amplification (LNA) and RF Bandpass Network (BPF)
To obtain better frequency selectivity, the Rx bandpass utilizes multiple electrically tunable circuits.
The block diagram is shown below: