MT312CSLASHCG datasheet(16/90 Pages) ZARLINK

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MT312 Functional Overview

Figure 12 - MT312 Control Structure

Command

Driven

Control

Acquisition/

Track

State machine

QPSK

MT312

format

registers

High level input/output

(MBaud, MHz)

Low level register read/write

1.5.2 Symbol Rate and Code Rate Search Mode

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.

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

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.

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.

MT312C/CG Datasheet(PDF) 16 Page - Zarlink Semiconductor Inc

MT312C/CG Datasheet(HTML) 16 Page - Zarlink Semiconductor Inc

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