Kenwood TK-2102G Service Manual page 8

TK-2102G
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 (D1 and
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, R1,
and C1 causes the voltage applied to the UL pin of the mi-
croprocessor 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 amplifi ed
by IC500 (1/2), passes through a pre-emphasis circuit, and
amplifi ed by the other IC500 (1/2) to perform IDC operation.
The signal then passes through a low-pass filter (splatter
fi Iter) (Q501 and Q502) and cuts 3kHz and higher frequen-
cies. 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
8
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)
PRE-EMPHASIS
IDC
Fig. 6 Transmit audio and QT/DQT encoder
D2 in transmit mode and D3 and D4 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 trans-
mit mode. The outputs from Q3 and Q4 are amplifi ed by Q6
and sent to the buffer amplifi ers.
D1, 2
LPF
D3, 4
IC1
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 characteris-
tic frequency or higher, modulation is performed at the VCO
side by adjusting the balance. (See Fig. 6)
Q501, 502
LPF
(SPLATTER FILTER)
IC403
QT/DQT
TO
MUTE
Q4
TX VCO
RF AMP
Q6
BUFF AMP
Q3
RX VCO
Q5, 7
T/R SW
5C
R1
IC403
D7
UL
C1
MPU
VR501
MAX
D5, Q4
DEV
VCO
VR500
BALANCE
X1, IC1
REFERENCE
OSC
(TCXO)
Q2