L64780 LSI Logic Corporation, L64780 Datasheet - Page 28

• Manufacturer Part Number: L64780
• Description: Dvb-t Cofdm Demodulator Technical Manual 2/00
• Manufacturer: LSI Logic Corporation

2.7 Automatic Frequency Control (AFC)
2.7.1 Analog Frequency Synchronization
2.7.2 Digital Frequency Synchronization
2-10
The two methods for applying the correction required to achieve
frequency synchronization are analog and digital. The method used
depends on the design of the analog tuner. The L64780 allows both
options, which are described in the following subsections.
This block adjusts the frequency of the local oscillator in the tuner so that
the center frequency of the low IF is equal to its nominal value of f
The adjustment must be accurate to within a small fraction of the carrier
spacing, so that intercarrier interference is kept to an acceptably low level
and the tracking ability of the channel estimation is not absorbed in
correcting for the error in the frequency of the local oscillator.
The block takes the sequence of complex numbers from the output of the
FFT block and produces a control signal for the local oscillator. This
control signal is delivered as a single-wire Sigma-Delta (
The initial error in the frequency of the local oscillator can be several
times the carrier spacing; the measurement of the frequency error relies
on the continual pilot carriers. The measurement range is
the 8k mode and
is derived from the measured frequency error.
The value of the control signal can also be read and initialized through
the microprocessor interface, which allows a previously stored value to
be used when a channel is selected, making acquisition faster.
This block uses digital signal processing (DSP) to correct for the
tolerance in the frequency of the local oscillator in the tuner. The DSP
shifts the frequency of the complex baseband signal so that its center
frequency is zero. The shift needs to be accurate to within a small
fraction of the carrier spacing so that intercarrier interference is kept to
an acceptably low level and so the tracking ability of the channel
estimation is not absorbed in correcting for the error in the frequency of
the local oscillator.
Architectural Overview
143 kHz in the 2k mode. The oscillator control signal
) signal.
51.7 kHz in
LIF
.