Kenwood TK-361SA Service Manual page 9

■ Receive Signaling
• QT/DQT
The output signal from FM IC (IC201) enters the MCU
(IC405) through IC301. IC405 determines whether the QT or
DQT matches the preset value, and controls the SP MUTE
and the speaker output sounds according to the squelch re-
sults.
3. PLL Frequency Synthesizer
The PLL circuit generates the fi rst local oscillator signal
for reception and the RF signal for transmission.
■ PLL
The frequency step of the PLL circuit is 5 or 6.25kHz.
A 12.8MHz reference an oscillator signal is divided at IC1
by a fi xed counter to produce oscillator (VCO) output signal
which is buffer amplifi ed by Q2 then divided in IC1 by a pro-
grammable counter. The divided signal is compared in phase
with the 5 or 6.25kHz reference signal from the phase com-
parator in IC1. The output signal from the phase comparator
is fi ltered through a low-pass fi lter and passed to the VCO
to control the oscillator frequency. (See Fig. 4)
■ 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, D4,
D6 and D7 in transmit mode and D3, D5, D8 and D9 in re-
ceive mode). The RX pin is set high in receive mode causing
Q5 turn on.
The TX pin is set high in transmit mode. The outputs
from Q3 and Q4 are amplifi ed by Q6 and sent to the RF am-
plifi ers.
D2,4
LPF D6,7 TX VCO
LPF
D3,5
D8,9 RX VCO
5kHz/6.25kHz
1/N
PLL DATA
REF OSC
1/M
5kHz/6.25kHz
X1
12.8MHz
Fig. 4 PLL circuit
CIRCUIT DESCRIPTION
Q2
Q4
RF AMP
Q6
BUFF AMP
Q3
Q9
RF AMP
RX
Q5,7
T/R SW
TX
Charge
Phase
comparator pump
IC1
PLL IC
■ Unlock Detector
If a pulse signal appears at the LD pin of IC1, an unlock
condition occurs, and the DC voltage obtained from C4, R5,
and D1 causes the voltage applied to the MCU (IC405) to go
low. When the MCU detects this condition, the transmitter
is disabled, ignoring the push-to-talk switch input signal.
4. Transmitter System
■ Microphone Amplifi er
The signal from the microphone passes through IC301.
The signal passes through the Audio processor (IC301)
for the maximum deviation adjustment, and goes to the
VCO modulation input.
AGC
MIC
IC301
AQUA-L
LPF
BEEP
IC405
MCU
Fig. 5 Microphone amplifi er
■ Drive and Final Amplifi er
The signal from the T/R switch (D101) is amplifi ed by the
pre-drive (Q101) to 50mW.
The output of the drive amplifi er is amplifi ed by the RF
power amplifier (Q102 and Q103) to 2W (1W when the
power is low). The RF power amplifi er consists of two MOS
FET stages. The output of the RF power amplifi er is then
passed through the harmonic fi lter (LPF) and antenna switch
(D103 and D122) and applied to the antenna terminal.
Q100 Q101 Q102
T/R SW RF Pre-drive Drive
(D101)
AMP AMP AMP
VD
VG
5T 5T
R127
+B
R128
R129
PCTV
(IC405)
Fig. 6 Drive and fi nal amplifi er and APC circuit
■ APC Circuit
The APC circuit always monitors the current flow-
ing through the RF power amplifi er (Q102 and Q103) and
keeps a constant current. The voltage drop at R127, R128
and R129 is caused by the current fl owing through the RF
power amplifi er and this voltage is applied to the differential
amplifi er IC101 (1/2).
TK-361SA
VCO
QTVCO LPF
X1
QTTCXO LPF TCXO
ANT
Q103 D103,122
Final ANT
LPF
AMP SW
VG
VG
VDD
IC101 IC101
(1/2) (2/2)
9