Kirisun FM540, Service Manual

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FM540 Service Manual
signal (51.65MHz) in IC6 to generate the second IF signal (450kHz). The second IF signal will be amplified
and limited in IC700 and after being filtered by CF1 crystal filter (450kHz), it will be demodulated by IC700
and audio signal will be output.
•
Squelch Circuit
The signal demodulated by IC6 is sent to the noise amplifier, and the amplified noise signal is sent to Q21
for further amplification and to D21 for wave detection to generate DC electric level, which is sent to the
MCU control squelch circuit. The voltage is inversely proportional to the input signal.
3.4 TX Principle
Figure 3-3 Power Amplification and Antenna Schematic Diagram
The modulated RF signal from VCO is sent to Q600 for power amplification after being amplified by Q1, Q2,
and Q5.
IC4 is controlled by MCU APC circuit to change the grid bias voltage, easily controlling the output power of
the transmitter.
•
APC (automatic power control) Circuit
D9 and D10 are standing wave current detection. IC4 is power amplifier.
If the output power of the transmitter is too high, the power amplifier current increases and the output
voltage from D9 and D10 will go up, which causes the VGG offset voltage on IC1 decreases and thus the
output power of the transmitter decreases. Vice versa. In this way, the output power of the transmitter stays
stable in different working environment.
MCU sets the power by changing the voltage input to IC4.
8T
SW
D2
DRIVE AMP
Q5
FINAL AMP
IC1
ANT SW
D3,D37
LPF
APC
IC4
APC
8T
8T
8T
D9 ,D10
RX BPF
VGG
VSWR
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