The L64733 directly accepts the RF In L-Band signal input from the
satellite LNB feed. The L64733 handles a fully loaded raster of
transponder signals from 925 MHz to 2175 MHz. RF In is internally
matched to 75
Ω,
and except for a DC-blocking capacitor, requires no
matching network between the cable connector and the L64733 input
pins. The L64733 uses the L64734 INSEL signal to select the
appropriate RF function (Normal or Loop-Through mode).
The RF signal goes to a variable gain stage, which is controlled by the
L64734 AGC1 signal. The L64734 adjusts AGC1 in conjunction with
AGC2 to maximize the SNR of the RF signal while maintaining proper
levels at the baseband outputs (IOUT and QOUT). The signal then goes
to two mixers in the quadrature demodulator. The mixers are fed with
local oscillator signals that are offset by 90 degrees from one another.
The quadrature demodulator performs a direct frequency conversion of
the RF signal to baseband while splitting the signal into quadrature I and
Q signal paths.
The baseband signals pass through a pair of variable-gain amplifiers that
are controlled by the AGC2 pin, which, in turn is controlled by the
L64734. The signals are then filtered through a pair of 7th-order filters
for anti-aliasing. The filter shape is 7th-order Butterworth, followed by a
single-pole delay equalizer. The filter cut-off frequency, which is
controlled by the L64734 FLCLK signal, is related to the baud rate. The
filtered baseband output signals go to the differential output stages at
IOUTp, IOUTn, QOUTp and QOUTn.
The baseband outputs of the L64733 go to the L64734, where they are
digitized by the analog-to-digital converter (ADC). The outputs then go to
a BPSK/QPSK demodulator, where they are filtered. The demodulator
then sends them to the L64734 FEC decoder pipeline, which outputs an
MPEG-2 transport stream.
The frequency synthesizer functionality is split between the L64733 and
L64734. The Synthesizer Control Module resides on the L64734; it
generates control signals for the L64733 Tuner IC frequency synthesizer.
The Synthesizer Control Module also contains some of the
programmable counters that are part of the synthesizer feedback loop.
The L64733 contains many of the analog functions of the frequency
synthesizer, as well as the RF local oscillator and crystal reference
oscillator. Tuning oscillator signals are generated for the mixers in the
4
L64733/L64734 Tuner and Satellite Receiver Chipset
LSI [ LSI COMPUTER SYSTEMS ]
