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D3, 5, 7, 8
LPF
LPF
D4, 6, 9, 10
PLL IC IC1
1N
PLL DATA
REF OSC
1M
X1
16.8MHz
Fig. 6 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 C19,R6
and Q1 causes the voltage applied to the microprocessor
to go high. When the microprocessor detects this condition,
the transmitter is disabled, ignoring the push-to-talk switch
input signal. (See Fig. 7)
IC1
R16
LD
C19
PLL IC
Fig. 7 Unlock detector circuit
4. Transmitter System
1) Microphone Amplifier
The signal from the microphone passes through the IC601.
When encoding DTMF, it is turned OFF for muting the
microphone input signal by IC601.
The signal passes through the Audio processor (IC601) for
the maximum deviation adjustment, and goes to the VCXO
modulation input.
CIRCUIT DESCRIPTION
Q6
Q7
TX VCO
DOUBLER
Q9
Q11
BUFF AMP
RF AMP
Q5
RX VCO
Q8, 12
T/R SW
RX
TX
5kHz/6.25kHz
PHASE
CHARGE
COMPARATOR
PUMP
5kHz/6.25kHz
5C
IC805
R6
R7
Q1
UL
CPU
IC601
AGC
MIC
AQUA
LPF
IC805
DTMF/2 TONE
CPU
Fig. 8 Microphone amplifier
2) Drive and Final Amplifier
The signal from the T/R switch (D201 is on) is amplified by
the drive amplifier (Q207) to 50mW.
The output of the drive amplifier is amplified by the RF power
amplifier (Q211) to 5.0W (1W when the power is low). The
RF power amplifier consists of two MOS FET stages. The
output of the RF power amplifier is then passed through
the harmonic filter (LPF) and antenna switch (D204 and
D206) and applied to the antenna terminal.
Q206
Q207
From
Pre-DRIVE
DRIVE
T/R SW
AMP
AMP
(D201)
VG
VG
+B
R264
R265
R270
IC201
PCTV
(IC805)
Fig. 9 Drive and final amplifier and APC circuit
3) APC Circuit
The APC circuit always monitors the current flowing through
the RF power amplifier (Q211) and keeps a constant current.
The voltage drop at R264, R265 and R270 is caused by the
current flowing through the RF power amplifier and this
voltage is applied to the differential amplifier IC201(1/2).
IC201(2/2) compares the output voltage of IC201(1/2) with
the reference voltage from IC805. The output of IC201(2/2)
controls the VG of the RF power amplifier, drive amplifier
and pre-drive amplifier to make both voltages the same.
The change of power high/low is carried out by the change
of the reference voltage.
TK-2168
VCO
IC606
BUFFER
LSDVCO
LPF
X1
LPF
LSDTCXO
TCXO
D204
Q211
D206
RF
ANT
LPF
POWER AMP
SW
VG
VDD
IC201
(1/2)
(2/2)
ANT
9
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