HYT TC-618, Руководство по обслуживанию

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TC-618/626 Service Manual
13
amplification. Then the amplified signals feed to the three-stage bandpass circuit (mainly composed of
D505, D506, D507 and peripheral components) to effectively reject out-of-band interference and to
obtain pure RF signals. Finally, the signals go to the mixer circuit.
MCU provides the electrically tunable control signal (its level can be obtained via table lookup or
calculation), making the varactor operate within an appropriate voltage range. It forms a bandpass filter
with the peripheral inductors/capacitors. The capacitance of the varactor changes along with the control
voltage of MCU, making the center frequency of the bandpass filter circuit change. In this way, the
requirements of preset Rx sensitivity and out-of-band rejection are satisfied.
2) Mixer Circuit and BPF Circuit
The block diagram of the mixer circuit is shown in the figure below:
L530 22nH
C533
C534
C532
C531
XF1
Q501
38.85MHZ
1stIF BPF
RF MIXER
AN29160
C528
Figure 4
The mixer circuit mainly mixes the local oscillator signal from VCO with the received RF signal, to
provide the first IF signal 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
feed 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:
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
3
1
R431
R420
R439
R500
C440 C439
to RF PA(APC)
C434
R435
to BPF(TUNE)
Figure 5