HYT POWER446, Service Manual

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POWER446 Service Manual
9
peripheral components) to effectively eliminate out-of-band interference and to obtain pure RX RF
signals. Finally, the signals feed to the mixer circuit.
The electrically tunable control signal (its level can be obtained by searching the corresponding table, or
calculated by the formula) is provided by MCU, so that the varactor can operate within an appropriate
voltage range. It forms a bandpass filter with the peripheral inductors/capacitors, and changes with the
MCU control voltage so as to satisfy the requirements for preset RX sensitivity and out-of-band rejection.
2) Mixer and IF Bandpass Filter Circuit
Figure 4
This circuit applies active mixer to mix the local oscillator signal from VCO with the RX RF signal, to
provide the first IF signal for frequency discrimination by the demodulator chip.
The mixer Q501 employs a dual-gate MOSFET (3SK318) for better noise-control and square-law
performance, as well as high isolation degree between the local oscillator signal and the RX signal. In
addition, the mixer has a certain gain (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, which employs a first-class crystal filter to assure sufficient bandwidth and good selectivity.
Then the signals go to the baseband processor (AN29160) for demodulation.
4. Automatic Power Control 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
3
1
R431
R420
R439
R500
C440 C439
to RF PA(APC)
C434
R435
to BPF(TUNE)
Figure 5