TC-268/368 Service Manual
12
2) VCO
The operating frequency is generated by Q4 in transmit mode and Q3 in receive mode. The oscillator
frequency is controlled by applying the VCO control voltage, obtained from the phase comparator, to the
varactor diodes (D2 and D4 in transmit mode and D1 and D3 in receive mode). The T/R pin is set high
in receive mode causing Q5 and Q7 to turn Q4 off, and turn Q3 on. The T/R pin is set low in transmit
mode. The outputs from Q3 and Q4 are amplified by Q6 and sent to the buffer amplifiers.
3) UNLOCK DETECTOR
If a pulse signal appears at the LD pin of IC1, an unlock condition occurs, and the DC voltage
obtained from D7, R6, and C1 causes the voltage applied to the UL pin of the microprocessor to go low.
When the microprocessor detects this condition, the transmitter is disabled, ignoring the push-to-talk
switch input signal. (See Fig. 5)
IC1
LD
PLL IC
D7
5C
R6
C1
IC403
UL
MPU
Fig. 5. Unlock detector circuit
4. Transmitter
1) Transmit audio
The modulation signal from the microphone is amplified by IC500 (1/2), passes through a
pre-emphasis circuit, and amplified by the other IC500 (1/2) to perform IDC operation. The signal then
passes through a low-pass filter (splatter filter) (Q501 and Q502) and cuts 3kHz and higher frequencies.
The resulting signal goes to the VCO through the VCO modulation terminal
for direct FM modulation. (See Fig. 6)
2) CTCSS/CDCSS encoder
A necessary signal for CTCSS/CDCSS encoding is generated by IC403 and FM-modulated to the
PLL reference signal. Since the reference OSC does not modulate the loop characteristic frequency or
higher, modulation is performed at the VCO side by adjusting the balance. (See Fig. 6)
VCO
D5.Q4
VR500
BALANCE
VR501
MAX
DEV
X1.IC1
PEFERENCE
OSC
CTCSS/CDCSS
TO
TTS
TTS
MUTE
Q503
SW
MIC
IC500(1/2)
MIC
AMP
PREEMPHASIS
IC500(1/2)
IDC
Q501.502
LPF
(SPATTER FILTER)
IC403
Fig. 6. Transmit audio CTCSS/CDCSS
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