Kenwood TK-2107 Service Manual page 7

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
PLL DATA
REF OSC
12.8MHz
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)
4. Transmitter
1) Transmit audio
The modulation signal from the microphone is amplified by
IC500 (1/2), passes through a preemphasis circuit, and
amplified by the other IC500 (1/2) to perform IDC operation.
The signal then passes through a low-pass filter (splatter fiIter)
(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)
IC500 (1/2)
MIC
AMP
MIC
Q503
SW
TK-260 :K, K2
CIRCUIT DESCRIPTION
PLL IC IC1
5kHz/6.25kHz
I/N
PHASE CHARGE
COMPARATOR PUMP
I/M
5kHz/6.25kHz
Fig. 4 PLL circuit
IC500 (1/2)
PREEMPHASIS
IDC
MUTE
Fig. 6 Transmit audio QT/DQT
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.
Q4
D1, 2
TX VCO
LPF
Q3
D3, 4
RX VCO
IC1
D7
LD
PLL IC
Fig. 5 Unlock detector circuit
2) QT/DQT encoder
A necessary signal for QT/DQT 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)
Q501, 502
VR501
LPF
MAX
(SPLATTER FILTER)
DEV
IC403
QT/DQT
TO
TK-2107
Q2
RF AMP
Q6
BUFF AMP
Q5, 7
T/R SW
5C
R6
IC403
UL
C1
MPU
D5, Q4
VCO
BALANCE
X1, IC1
REFERENCE
OSC
(TCXO)
7