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Service Manual
8. VHF (136-174 MHz) Information
8.1 Transmitter Circuit
The transmitter circuit mainly consists of:
RF power amplifier circuit
Low-pass filter circuit (for suppressing harmonics)
Auto power control circuit (APC) (including temperature detection circuit)
π Attenuator
Pre-driver 1
TX VCO
8.1.1 RF Power Amplifier Circuit
The carrier signal generated by the TX VCO is modulated and amplified, and then feeds to
the transmitter circuit via the following steps:
Step 1 The signal passes through a π-shaped attenuator first, allowing interstage isolation
between the RF power amplifier circuit and the TX VCO.
Step 2 The signal goes to a pre-driver amplifier (U8004) for pre-amplification, providing
further interstage isolation.
Step 3 The signal goes to a pre-driver amplifier (Q8006) for adequate power amplification,
so as to obtain further amplification in the driver amplifier (Q8005).
Step 4 After being processed by multiple amplifiers, the signal is processed by a microstrip
matcher at the output of the final PA (Q8004) to complete output impedance matching,
as so to reduce output power loss due to impedance mismatch.
Step 5The signal passes through the TX/RX switch (D8004) and goes to the low-pass filter.
Pre-driver 2
Drive-stage
Final-stage
Gate Voltage Control
I/V Convertor
BATT+
Figure 8-1 Diagram of Transmitter Circuit
8. VHF (136-174 MHz) Information
TX/RX Switch
Microstrip Matcher
TV1/APC
Subtracter
TEM Sensor
Near To Final PA
232
ANT
Low-Pass Filter
To RX
To GPS
Tem Det Level
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