Low-Pass Filter Circuit; Auto Power Control Circuit - RCA RDR9000 Service Manual

Service Manual 6. UHF1 (400-470 MHz) Information

6.1.2 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.

6.1.3 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 then converts it to DC voltage. Then the DC voltage is compared with the
voltage corresponding to the protection temperature (generally 80% of the extreme
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.
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