TC-618/626 Service Manual
12
The switching diode Q652 performs switching between transmission and reception under the control of
T/R: in Tx mode, T/R is set to low level and Q653 becomes conductive, making Q100 operate; in Rx
mode, T/R is set to high level and Q653 is cut off, making Q101 operate. The output from Q100 and
Q101 is amplified by Q102 and then feeds to the buffer amplifier for further processing.
If the PLL is unlocked, the LD pin of U202 outputs low level. If the microprocessor detects such situation,
Tx/Rx operation is prohibited, and the alarm alert sounds.
2. RF Power Amplifier Circuit (Tx Section)
The block diagram of the RF power amplifier circuit is shown in the figure below:
R/TSW
L453
1u
RX
RQA0002
2
1
3
3 Amplifier
Q401
2SC4988
1
2
3
APC
LPF
2SK3475
2
1
3
Q400
2SC5108
1
2
3
Q104
2SC5108
1
2
3
MATCH
ANTENNA
Q402
0
RF PA
Q403
Figure 2
To obtain the required RF power, the RF signal from VCO goes to the buffer Q104 and to the driver
amplifiers Q400 and Q401. Then the amplified RF signal feeds to the driver Q402, which pre-amplifies
power of the input signal to drive the final-stage power amplifier Q403. The input RF signal is then further
amplified by Q403, and feeds to the LC low-pass filter (LPF) through the Tx/Rx switching diode D401.
Finally, it is transmitted from the antenna after ultroharmonics are removed.
3. Rx LNA and Mixer Circuit (Rx Section)
1) Rx LNA and RF BPF
To allow better frequency range selectivity, the Rx bandpass filter adopts the multi-stage electrically
tunable circuit, as shown in Figure 3.
C505
LNA
TUNE BPF
TUNE FROM MCU
C504
C523
L508
L506
Q500
D503
L507
D506
1
2
3
L501
C526
C519
C524
L502
C521
C506
TUNE FROM MCU
D504
D505
D507
TUNE BPF
Figure 3
After out-of band signals are removed by the electrically tunable bandpass circuit (D503, D504, L501,
L502, C503, C505 and C507), the signals from the antenna go to the low-noise amplifier (LNA) Q500 for
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