User's Manual

PRELIMINARY
PRELIMINARY PRELIMINARY
PRELIMINARY –
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– Service
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Service Manual
Manual Manual
Manual
DR• CHKD• APPD•
33
Fig. 7. APC system
9
）
ANT switch and LPF
The RF amplifier output signal is passed through a low-pass filter network
and a transmit/receive switching circuit before it is passed to the antenna
terminal. The transmit/receive switching circuit is comprised of D101, D102
and D103. D102 and D103 turned on (conductive ) in transmit mode and
off (isolated) in receive mode.
10
）
APC
The automatic power control (APC) circuit stabilizes the transmitter output
power at a predetermined level by sensing the drain current of the final
amplifier Field Effect Transistor (FET). The voltage obtained from the
above drain current with a reference voltage which is set using the
microprocessor. An APC voltage proportional to the difference between
the sensed voltage and the reference voltage appears at the output of
IC100 (1/2). This output voltage controls the gate of the FET power
amplifier, which keeps the transmitter output power can be varied by
the microprocessor which in turn changes the reference voltage and
hence , the output power.
11
）
Terminal protection circuit
When the thermistor (TH102) reaches about 80℃•the protection circuit