MC92314 Datasheet, PDF (83/100 Pages) Motorola, Inc – DVB-T Single Chip Demodulator Application Note

Freescale Semiconductor, Inc.
Usage and Performance of Motorola’s Single-chip DVB-T Device
5.4.1.1.1 Changing AFC Sweep Start
The AFC loop (described in paragraph 3.2.2.2) can be controlled by setting the parameters used
during the initial sweep during acquisition. It is possible to adjust the speed as well as the starting
point of the sweep (refer to paragraph 4.2.2.1.7 and paragraph 4.2.2.1.10). The current position
of the AFC is reported in the AFC Feedback register (see paragraph 4.2.2.1.14). This number
gives an indication of the LO offset in the tuner w.r.t. the center frequency of the current RF
channel.
To shorten the sweep time it is possible to use this feedback value. During normal operation or
just before a channel change the value should be read by system controller and stored. As the
LO offset maybe slighthly different for different RF channels, the new center frequency can be
taken into account together with the feedback value to calculate an expectation for the new LO
n ^offset.
tio ^{Depending from the usual sweep direction this value should be decreased (upward sweep) or}
a ^{increased (downward sweep), e.g. by app. $120 for a deviation of 10 KHz. If the new position is}
close to the lower edge of the range it may be useful to chose downward sweep and to increase
m _{the number.}
for This results in the following recommended procedure associated with a channel change:
In _{• Read the AFC Feedback register and calculate the expectation for the new channel.}
y _{• Program the tuner to the new channel, observe possible offsets in frequency.}
ar • Program the AFC start value in the appropriate register (2 byte I²C write).
in • Issue a soft reset for the OFDM part to force a new AFC sweep.
lim ^{These steps ensure that the AFC sweep starts near the point were the AFC circuit should find its}
final lock position. The deviation used must be searched by evaluating different distances,
e depending e.g. from the settling time of the tuner, the precision of the tuner LO or the other
Pr ^{components that are controlled by the OFDM block like VCXO or AGC amplifier.}
Note that changing the AFC sweep start may have no effect in poor reception conditions. The
reason for this is that in these cases several sweeps by the AFC circuit may be necessary. The
value stored in the sweep start register is used only after a soft reset. If the sweep comes to the
end of its range it starts at the opposite end instead of the sweep start position. This prevents
unintentional conditions were lock can never be achieved because the position the AFC is
looking for is outside of the sweep range.
5.4.1.1.2 Changing AGC Integrator Gain
Additional increase in acquisition speed may be possible by changing the gain of the AGC
integrator during this phase. This loop defaults to be stable in all circumstances to allow for
resolving the amplitude differences of 64-QAM. The default value (2’s complement number) for
normal operation is a small negative number. During the acquisition phase it may be tolerable to
set it to a positive value (app. $4) to increase the speed of the AGC control signal.
Single Chip DVB-T Demodulator - Rev. 1.3 (11/30/98)
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