Service Manual VHF (136-174MHz) Information
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Step 4 After processed by multiple amplifiers, the signal passes through an impedance matching circuit,
so as to reduce output power loss due to impedance mismatch.
Step 5 The signal passes through the TX/RX switch and goes to the low-pass filter.
11.1.2 Low-pass Filter Circuit
The low-pass filter is a high-order Chebyshev filter composed of lumped-parameter inductors and
capacitors. Via this filter, the spurious signal within the stop band can be attenuated as much as
possible while the in-band ripple is within the required range.
11.1.3 Auto Power Control Circuit
In the auto power control and temperature detection circuit, the drain current from the driver amplifier
and final-stage amplifier is converted to voltage via the sampling resistor and subtraction circuit
(composed of the first operational amplifier).
This voltage is compared with the APC control voltage (output by DAC) at the second operational
amplifier. Then the error voltage, which is output by the second operational amplifier, controls TX
power by controlling the bias voltage at the gates of the amplifiers (including the driver amplifier and
the final-stage amplifier). The temperature sensor detects the surface temperature of the final-stage
amplifier, and converts it to DC voltage. Then the DC voltage is compared with the voltage
corresponding to the protection temperature (generally 80% of the extreme junction temperature) of
the amplifier. If the surface temperature is too high, the bias voltage of the amplifier will be reduced
until the surface temperature falls below the protection temperature.
11.2 Receiver Circuit
The receiver circuit mainly comprises the RF band-pass filter, low-noise amplifier, mixer, IF filter, IF
amplifier and IF processor.
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