Service Manual 6. UHF1 (400-470 MHz) Information
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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)
Pre-driver 2 Drive-stage
Final-stage
TX/RX Switch
Low-Pass Filter
ANT
Microstrip Matcher
TV1/APC
To RX
Gate Voltage Control
Subtracter
TX VCO
Sampling Resistor
BATT+
I/V Convertor
Pre-driver 1
π Attenuator
To GPS
TEM Sensor
Near To Final PA
Tem Det Level
Figure 6-1 Diagram of Transmitter Circuit
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.
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