Service Manual

12
is utilized.
The mixer tube (Q501) utilizes dual gate FET MOS (3SK318) and has better noise characteristics
and square law characteristics. The isolation between the local oscillator signals and the Rx signals
is high. To ensure proper sensitivity and certain gain for the mixer tube, tune delicately via the
offset.
The signals output from the mixer pass through inductor L509 to remove residual spurious and then
enter the first IF filter. The filter utilizes the first-class crystal filter to ensure sufficient bandwidth and
excellent selectivity. The signals finally enter baseband processing chip AN29160 for demodulation.
APC/TUNE Automatic Power Control Circuit
Block Diagram for the Circuit:
R430
R421
Q431
DTA114YE
R437
C441
R422
R441
R433
R434
from MCU
V_BAT
TX_VCC
R432
R440
C435
C437
R438
C436
R442
C438
-+
+-
U430
NJM2904
1
3
4
2
5
7
8
6
R436
Q430
DTC114EE
2
31
R431
R420
R439
R500
C440C439
to RF PA(APC)
C434
R435
to BPF(TUNE)
Figure 6
Subject to the selected channel, MCU outputs the corresponding PWM waveform, which is
reshaped by the RC filter network (R441, R442, C440 and C441) to derive the APC/TUNE control
signal level. One part of them is used as the control voltage of the electrically tunable circuit after
passing through R500. In transmitting mode, the levels are voltage-divided by R439 and R440 to
obtain the APC reference voltage.
The transmitting current passes through R420, R421 and R422 to derive the error detect voltage.
The voltage is amplified by operational amplifier IC U430 and then compared with APC reference
voltage to output APC control voltage and to form closed-loop negative feedback power control
when the transmitting current changes.