Mixer Circuit (Mixer) And If Bandpass Network (Bpf); Apc/Tune Automatic Power Control Circuit; Diagram For Mixer Circuit; Diagram For The Circuit - HYT TC-610 Service Manual

will be fed to the mixer stage.
MCU provides the electrically tunable control signal, the required level of which can be obtained through
table looking up or formula computing, to accurately control varactors to operate within proper voltage
range. The control signal forms a bandpass filter with the periphery inductors and capacitors and tracks
Rx frequency according to the change of MCU control voltage, to meet the requirements of preset Rx
sensitivity and out-of-band rejection.

2. Mixer Circuit (MIXER) and IF Bandpass Network (BPF)

Diagram for Mixer Circuit

AN29160
The mixer circuit mainly processes the local oscillator signal and Rx RF signal output by VCO. The first
IF from the mixer circuit is used by the demodulator to discriminate frequency. Here the active mixer is
utilized.
The mixer transistor (Q501) utilizes dual gate MOS FET (3SK318) and has better noise characteristics
and square law characteristics. The isolation between the local oscillator signal and the Rx signal is high.
To ensure proper sensitivity, the mixer transistor should have a certain gain, which can be tuned
delicately via offset.
The output signal from the mixer circuit passes through inductor L509 to remove residual spurious and
then enters the first IF filter, which utilizes the first-class crystal filter to ensure sufficient bandwidth and
excellent selectivity. The signal finally enters baseband processor AN29160 for demodulation.

APC/TUNE Automatic Power Control Circuit

Diagram for the Circuit:

R420
V_BAT
R421
R422
R433
TX_VCC
1stIF BPF
38.85MHZ
XF1
R430 R432
U430
R431 NJM2904
1
C437
2
- +
C438
3
+ -
R434 R435
4
2
R439
Q430
DTC114EE
Q501
RF MIXER
Figure 5
Q431
DTA114Y E
8
R436
7
R437
6
C436
5
R441
R440 C439 C440
R500
to BPF(TUNE)
Figure 6
14
TC-610/620 Service Manual
22nH L530
C531
C534
C532
C533
C528
R438
to RF PA(APC)
C434
C435
from MCU
R442
C441