MT312CSLASHCG datasheet(27/90 Pages) ZARLINK

27 / 90 page

Tuner Control

MT312

4.4.3 FR OFF: Frequency Offset. Register 38 (R/W)

FR OFF[7:0] Frequency offset correction value in MHz x 32. This 2’s complement 8 bit number represents an

offset from -4MHz to +3.96875MHz. Default value 0.

The frequency search is carried out in the range [(-FR LIM + FR OFF), (FR LIM + FR OFF]. Frequency offset

If the location of the wanted channel with respect to the current channel is known and if the synthesiser step

size is too large to set the precise frequency of that channel, then the FR OFF register can be used to take up

any residual frequency offset.

4.5 Tuner Control Read Registers

4.5.1 Measured LNB Frequency Error. Registers 7 - 8 (R)

LNB FREQ[15:0]: Once the chip is in lock these two registers provide a measurement of the frequency of the

signal at the input to MT312. Ideally, this frequency is zero. Due to LNB frequency uncertainty this frequency

may take a positive or negative value. Then the analogue front-end may be re-tuned to bring this offset close to

zero. Note that MT312 indicates the frequency location of the QPSK spectrum with respect to zero frequency.

The direction in which the synthesiser frequency has to be stepped depends on the design of the analogue

front-end. Also note that in many instances it will not be necessary to re-tune even when there is a relatively

large frequency offset. This is because MT312 compensates for this frequency offset before it demodulates the

signal. Re-tune only if a substantial part of the QPSK spectrum is affected by the SAW or base-band ﬁlter

which precedes MT312. This will be the case only for symbol rates which are close to the maximum symbol

rate supported by the above mentioned ﬁlters.

When MT312 locks part of the frequency offset is taken up by the frequency compensation mixer and part by

the carrier synchroniser. LNB FREQ gives only the value in the frequency compensation mixer. Over a short

period of about 1 s after lock, the carrier synchroniser will transfer all the frequency compensation to the mixer.

Hence the LNB FREQ reading will have an error less than ±5% of the symbol rate, during this short period after

lock. If an accurate frequency reading is needed immediately after lock, the calculation given in section on

FREQ ERR2 has to be performed by external software.

LNB FREQ[15:0] Frequency offset MHz x 512. This is a 2’s complement 16 bit number.

e.g. a hex value of

F680 (=-2432) represents an offset of -4.75MHz.

NAME

ADR

Def

hex

FR OFF

FR OFF[7:0] - Freq. Offset in MHz

R/W

NAME

ADR

Def

hex

LNB FREQ H

LNB FREQ15:8] Measured LNB frequency error (high byte)

LNB FREQ L

LNB FREQ[7:0] Measured LNB frequency error (low byte)

MT312C/CG Datasheet(PDF) 27 Page - Zarlink Semiconductor Inc

MT312C/CG Datasheet(HTML) 27 Page - Zarlink Semiconductor Inc

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