HYT TC-610P Service Manual page 16

The mixer circuit mainly mixes the local oscillator signal from VCO with the received RF signal, to
provide the first IF singal for frequency discrimination by the demodulator chip (active mixer is applied).
The mixer Q501 uses a dual-gate MOSFET (3SK318), and is characterized by good noise characteristic,
square-law characteristic, and high isolation degree between the first local oscillator signal and the Rx
signal. In addition, it also has a gain value (adjustable via offset), so as to assure sufficient sensitivity.
After residual spurious signals are removed by the inductor L509, the signals from the mixer go to the
first IF filter (first-class crystal filter assuring sufficient bandwidth and good selectivity). Then the signals
go to the baseband processor chip (AN29160) for demodulation.
4. APC/TUNE Circuit
The block diagram of the APC/TUNE circuit is shown in the figure below:
R420
V_BAT
R430
R421
R431
R422
R433
R434
TX_VCC
Q430
DTC114EE
MCU ouputs appropriate PWM waveform according to the selected channel. The waveform is then
rectified by the RC filter circuit (R441, R442, C440 and C441) to generate level of APC/TUNE control
signal. One flow of the signal goes to R500 to be used as control voltage of the Rx electrically tunable
circuit. In Tx mode, the signal goes to R439 and R440 for voltage division, and the APC reference
voltage is derived.
R432
U430
NJM2904
1 8
C437
- +
2 7
C438
+ -
3 6
R435
4 5
2
R439
Q431
DTA114Y E
R436
R437
C436
R441
R440 C439 C440
R500
to BPF(TUNE)
Figure 5
14
TC-610P Service Manual
R438
to RF PA(APC)
C434
C435
from MCU
R442
C441