Tuner Control MT312
4.4.3 FR OFF: Frequency Offset. Register 38 (R/W)
Def
hex
NAME
ADR
B7
B6
B5
B4
B3
B2
B1
B0
FR OFF
38
FR OFF[7:0] - Freq. Offset in MHz
R/W
00
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
register can be useful in reducing frequency search during channel hopping, especially with low symbol rates.
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)
Def
hex
NAME
ADR
B7
B6
B5
B4
B3
B2
B1
B0
LNB FREQ H
LNB FREQ L
07
08
LNB FREQ15:8] Measured LNB frequency error (high byte)
LNB FREQ[7:0] Measured LNB frequency error (low byte)
R
R
00
00
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.
27
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
