Dynex LC-22HV40 Service Manual page 23

3. MSD809
AGC Description
The AGC has two modes of operation. When the chip is powered up, the non-coherent IF AGC &
RF AGC will function first. The non-coherent AGC loop will provide the control voltage necessary for
proper gain control of the external gain amplifier and tuner in the RF section. The two AGC will
extend the dynamic range of power amplifier.
It automatically adjusts itself to any system with a matched filter. The automatic gain controller
(AGC) takes the input data stream from either the ADC output or the DC remover output, and
applies non-coherent detection of the gain level, and adjusts it to a pre-configured reference level.
The two AGC work exclusively in parallel; there is the AGC that feeds its output to the IF amplifier,
and the AGC that outputs to the tuner amplifier. The AGC also support manual mode, where one is
fixed to a pre-configured level, while the other performs normal AGC functions.
Demodulator Description
The first function of demodulator is to convert sampling clock to operation rate of two-times symbol
rate with interpolator. Interpolator generates operating rate clock of 21.52MHz from ADC rate of
24.69MHz with timing controller.
Demodulator also down-convert the received IF signal to near base-band signal. These functions
can compensate each estimated offsets from carrier recovery and symbol timing recovery.
Carrier recovery and symbol timing recovery estimate offsets perfectly in severe ghost environment.
Especially carrier recovery is able to track offsets in deep fading ghost environment with very weak
or no pilot tone.
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