TC-600 Service Manual
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low-pass filter and passed to the VCO to control the oscillator frequency. (See Figure 4)
Figure 4 PLL Circuit
2) VCO
The operating frequency is generated by Q16 in transmit mode and by Q8 in receive mode.
The operating frequency generates a control voltage through the phase compara tor to control
the varactor, so that the oscillator frequency is made consistent with the preset frequency in
CPU (D2 and D3 in transmit mode , D7 and D8 in receive mode). In receive mode, Q6 turns off
Q16 and turns Q7 on, which causes the T/R pin to be set to high level. The T/R pin is set to low level in
transmit mode. The output from Q8 and Q16 is amplified by Q14 and sent to the buffer amplifier.
4. Transmitter
1) Transmit Audio
The modulation signal from microphone is amplified by IC3, pre-emphasized, then filtered by another
low pass filter
（
separate filter
）
(Q25 and Q24) to eliminate the frequencies higher than 3KHz. The
resulting signal enters the VCO for direct FM modulation. The voice voltage amplitude control circuit
(Q46\R131\R601) is used to switch between wideband and narrowband. (See Figure 5)
2)
CTCSS/CDCSS Encoder
The signal needed by CTCSS/CDCSS encoder is generated by IC11 and FM-modulated to the PLL
reference signal. Since the reference oscillator can not modulate frequency without loop
characteristics, modulation is performed at the VCO side via the divider. (See Figure 5)
