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Service Manual
6. UHF1 (400–470 MHz) Information
6.1 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
6.1.1 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 5 The 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 6-1 Diagram of Transmitter Circuit
6. UHF1 (400-470 MHz) Information
TX/RX Switch
Microstrip Matcher
TV1/APC
Subtracter
TEM Sensor
Near To Final PA
29
ANT
Low-Pass Filter
To RX
To GPS
Tem Det Level
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