Kenwood TK-2202 Service Manual page 9

6) Squelch
Part of the AF signal from the IC enters the FM IC (IC201)
again, and the noise component is amplified and rectified
by a filter and an amplifier to produce a DC voltage
corresponding to the noise level.
The DC signal from the FM IC goes to the analog port of
the microprocessor (IC405). IC405 determines whether
to output sounds from the speaker by checking whether
the input voltage is higher or lower than the preset value.
To output sounds from the speaker, IC405 sends a high
signal to the SP MUTE line and turns IC302 on through
Q303,Q304,Q305,Q306 and Q316. (See Fig. 4)
7) Receive Signalling
(1) QT/DQT
The output signal from FM IC(IC201) enters the
microprocessor(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 results.
(2) MSK (Fleet Sync)
The MSK input signal from the FM IC goes to pin 31 of IC 301.
The signal is demodulated by MSK demodulator in IC 301.
The demodulated data goes to the CPU for processing.
RECEIVE SIGNALLING RECEIVE SIGNALLING
FM IF IC201
IF Amp
SIGNAL
DTMF
QT/DQT
IC405
QT/DQT IN
BUSY
CLK,DATA,
STD,LOADN
CPU
AF CONT
Fig. 4 AF amplifier and squelch
(3) DTMF
The DTMF input signal from the FM IC (IC201) goes to
IC301. The decoded information is then processed by the
CPU.
3. PLL Frequency Synthesizer
The PLL circuit generates the first local oscillator signal for
reception and the RF signal for transmission.
1) PLL
The frequency step of the PLL circuit is 2.5, 5, 6.25 or 7.5kHz.
A 12.8MHz reference oscillator signal is divided at IC1 by a
fixed counter to produce an oscillator (VCO) output signal
which is buffer amplified by Q2 then divided in IC1 by a
programmable counter. The divided signal is compared in
phase with the 5 or 6.25kHz reference signal from the phase
comparator in IC1. The output signal from the phase
comparator is filtered through a low-pass filter and passed
to the VCO to control the oscillator frequency. (See Fig. 5)
CIRCUIT DESCRIPTION
IC301
AQUA
IC302 Q306,316
AF PA SW
Q303,304,305
SW
TK-2202/2206
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 (D4 and
D7 in transmit mode and D5 and D9 in receive 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 amplified by Q6 and sent to the RF amplifiers.
LPF
LPF
PLL DATA
REF OSC
X1
12.8MHz
SP
Fig. 5 PLL circuit
3) 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 microprocessor
to go low. When the microprocessor detects this condition,
the transmitter is disabled, ignoring the push-to-talk switch
input signal.
4. Transmitter System
1) Microphone Amplifier
The signal from the microphone passes through the IC301.
When encoding DTMF, it is turned OFF for muting the
microphone input signal by IC301.
The signal passes through the Audio processor (IC301) for
the maximum deviation adjustment, and goes to the VCO
modulation input.
Q4
D4, 7
TX VCO
Q6
BUFF AMP
Q3
D5, 9
RX VCO
Q5, 7
T/R SW
PLL IC IC1
5kHz/6.25kHz
1N
PHASE CHARGE
COMPARATOR PUMP
1M
5kHz/6.25kHz
Q2
BUFFER
IC2
1/ 2
RX
TX
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