MT312 Functional Overview
High level input/output
(MBaud, MHz)
MT312
format
registers
Command
Driven
Control
Acquisition/
Track
State machine
QPSK
Low level register read/write
Figure 12 - MT312 Control Structure
1.5.2 Symbol Rate and Code Rate Search Mode
base-band, the pass-band of the low pass filter
should extend up to 30 MHz. However, it is
preferable to make this bandwidth larger by about 5
MHz, partly to reduce the in-band phase distortion
introduced by the filter and partly to reduce the loss
of signal due to LNB offset. The filter must attenuate
signals beyond 60 MHz by about 30 dB, as these
signal will alias to the useful frequency range with 90
MHz sampling.
Where the Symbol rate and/or the Viterbi code rate
are unknown, the MT312 can be programmed to
search for QPSK/BPSK signals. The user should
define the range(s) over which the search is
required. The MT312 will then locate and track any
signal detected. Failure to find a QPSK signal in the
specified frequency and specified symbol rate
ranges will be indicated by interrupts (see 6.2 QPSK
Demodulator Read Registers). MT312 will carry on
searching these ranges after issuing these
interrupts. When the MT312 has locked onto a
signal, the Symbol rate in MBaud may be read from
the MONITOR registers. The Viterbi code rate may
be read from the FEC STATUS register. This search
facility is primarily for the initial installation of a set
top box.
Although the system is designed for 45 MBaud, if the
actual symbol rate is much lower, say 1 MBaud, then
MT312 will automatically introduce all the digital
filtering needed to isolate the 1 MBaud signal.
Figure 13 - Alternative System Block Diagram -
SNIM6 shows an alternative application when a
reduced Symbol rate is acceptable. The SL1935
combines the functions of the RF pre-amp, direct
conversion zero IF tuner and synthesiser.
1.6 Direct Conversion Application
Figure 1 shows a direct conversion system that
mixes the L-band input to the tuner directly down to I
and Q baseband channels at zero intermediate
frequency.
The RF AGC amp and tracking filter provide the
required tuner noise figure and limit the total power
reaching the SL1925. These elements also give
isolation between the SL1925 local oscillator and the
L-band tuner input. This is an important factor since
both signals are at the same frequency.
The baseband filter is an anti-alias filter. This
replaces the filtering normally carried out with a SAW
filter in conventional single conversion tuners.
It is important to note that all the channel filtering
needed to isolate low Baud rate signals is contained
within the MT312. The low pass filter before MT312
is designed not to filter channels, but to minimise any
aliasing due to sampling. To illustrate this, let the
sampling frequency be 90 MHz and the maximum
symbol rate be 45 MBaud. The bandwidth of the 45
MBaud QPSK signal, with 0.35 roll-off, is about 60
MHz. If the channel has been mapped precisely to
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