L64704 LSI Logic Corporation, L64704 Datasheet - Page 130

• Manufacturer Part Number: L64704
• Description: Satellite Decoder Technical Manual 5/97
• Manufacturer: LSI Logic Corporation

5.9
Other Functions
5.9.1
Carrier Loop DC
Offset
Compensation
Equation 5.14
5-26
The following functions have been added in order to simplify the design
of the analog front end, to reduce the amount of external circuitry, and
to make the board layout as simple as possible.
As discussed in previous sections, filtering in the carrier loop is carried
out in the analog domain. Because of the imperfections of analog com-
ponents, the loop voltage could show a DC offset. This would have two
consequences:
For these reasons, the L64704 implements an internal offset compensa-
tion in the carrier loop. This internal compensation operates in two
modes: acquisition and tracking.
5.9.1.1 Carrier Acquisition Phase
In the acquisition phase, the L64704 reads the DC offset estimation from
the CAR_OFFSET[7:0] register (Group 4, APR 15) and subtracts it from
the loop voltage. The CAR_OFFSET register is assumed to have been
previously loaded by the microcontroller. The CAR_OFFSET parameter
is a signed integer ranging from -128 to +127.
Demodulator Module Functional Description
CAR OFFSET
During the acquisition phase, the frequency sweep becomes asym-
metric. The DC offset slows down the sweep in one direction and
makes it faster in the other direction. If the sweep is too slow, false
locks can occur at high SNR. If the sweep is too fast, locks can be
missed at low SNR.
During the tracking phase, the static error of the loop is not minimum.
This is because the DC offset generates a frequency ramp that the
loop has to compensate for.
T
If T
CAR_OFFSET = 0 and CAR_OPEN = 1, open loop), then the
CAR_OFFSET value can be computed by the following formula:
The
polarity is normal; select - if it is swapped.
up
up
can be measured by using an oscilloscope.
is the ramp-up time and T
choice depends on the polarity of the N/P outputs. Select + if
=
CAR SWR
------------------------------- -
2
dwn
T
------------------------------ -
T
dwn
dwn
the ramp-down time (with
–
+
T
T
up
up