Kenwood 144MHz FM TRANSCEIVER K2E Service Manual page 8

TH-K2AT/K2E/K2ET
Transmitter System
Microphone Amplifier Circuit
The audio signal from the microphone passes through a
high-pass filter (IC5) and enters a microphone amplifier (IC16).
The signal passes through a pre-emphasis circuit, limiter
amplifier and splatter filter inside the microphone amplifier.
The splatter filter will remove distortion outside the audio band.
MIC IC5
6
HPF
Pre-
emphasis
Fig. 5 Microphone amplifier circuit
Modulation Circuit
The audio signal amplified by the microphone amplifier
(IC16) passes through a semi-fixed volume (VR5) for
modulation adjustment, and goes to the VCO modulation
varicap (D6) for variable reactance phase modulation.
Drive and Final Circuit
The output signal from VCO (Q1) passes through an RF
amplifier (Q2, Q5, Q9) and drive amplifier (Q10, Q11), and is
amplified by a power amplifier (Q12).
Transmission/Reception Switching Circuit
The signal amplified by the power amplifier (Q12) passes
through a transmission/reception switching circuit and a low-
pass filter, and is fed to the antenna.
Q11
Q10
Q15
5T
B
8
CIRCUIT DESCRIPTION
IC16
MIC AMP
Limiter
1
VCO
Splatter
filter
Q12
Q58
APC AMP
1
AOUT
2
A–IN
3
A+IN
4
GND
Q13
B
Fig. 7 APC and Temperature protection circuit
The transmission/reception switching circuit consists of D10,
L19, D12 and D13. These diodes are turned ON in transmit
mode and OFF in receive mode to switch signals.
MIC IC5 IC16
MIC
HPF
AMP
DRIVE AMP
Q10
DRIVE AMP
Fig. 6 Modulation, Drive, Final and
Transmission/reception switching circuit
APC Circuit
The APC (Automatic Power Control) circuit is used to obtain
a stable transmission power and controls transmission output
by detecting the drain current of Q11 and Q12.
The transmission output can be changed in three levels:
High, Mid, and Low.
The reference voltage is output from the BPF/APC terminal
(pin 6) of the MPU (IC8) and the detection voltage generated by
R70, R71 and R72 are fed to the APC differential amplifier (IC2).
The voltage in proportion to the difference between
reference voltage and detection voltage is output from the
BOUT terminal (pin 7) of IC2 as an APC voltage.
D10
IC2
8
V+
Q14
7
BOUT
6
B–IN
5
B+IN
Q2
Q1 RF AMP
VCO
VR5
RF AMP
MAX DEV
Q11 D10,L19
D12,D13 LPF
ANT
SW
Q12
POWER AMP
ANT
LPF
RX
7
6
Q9
RF AMP
Q5
ANT
Q55
51
THERM
KAPC
BPF/APC
16
FINAL
IC8
MPU