L64704 LSI Logic Corporation, L64704 Datasheet - Page 120

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

5.6.1.5 Phase Lock Detection
Once the VCO frequency is close enough to the frequency of the
incoming wave, the signal lies in the pull-in range of the phase-locked
loop. When the loop is phase locked, the phase lock detector sets the
CAR_LC bit (Group 3, APR 9) to 1. To stop the sweep, the microcontrol-
ler must then set the CAR_SW bit (Group 4, APR 33) to zero.
The phase lock detector uses an internal threshold and an estimation
period, which are programmable using the CAR_THSL register (Group 4,
APR 27) and the CAR_OUT_SEL bit (Group 4, APR 33), respectively.
The CAR_OUT_SEL bit selects between a long and a short estimation
period. For operation at low Eb/No (less than 10 dB), the long period
should be selected (CAR_OUT_SEL = 0). A typical value of CAR_THSL
is then 31.
For operation at higher Eb/No (10 dB or higher), the short period can be
selected. This provides for a faster lock detection. In this case, a typical
value for CAR_THSL is 72.
5.6.1.6 Frequency Lock Detection
The frequency lock detector also has a configurable estimation period,
selectable by the same parameter as for phase lock detection (the
CAR_OUT_SEL bit). No threshold values have to be programmed for the
frequency lock detector; the thresholds are fixed and hard-coded in the
L64704.
5.6.1.7 False Locks
The microcontroller must take particular care to handle a false lock
condition correctly. A false lock occurs when phase lock has been
detected but the correct central frequency has not been reached yet.
This situation happens in QPSK for frequency offsets that are multiples
of 1/4T, where T is the QPSK symbol duration.
This case is detected by CAR_LC = 1 and CAR_LCF = 0 (both bits are
in Group 3, APR 9). When the microcontroller detects this situation, it
should set the CAR_OPEN (Group 4, APR 33) bit to 1 and reset it after
CAR_LC = 0. This has the effect of opening the carrier loop, forcing the
loop to run out of the false lock point.
5-16
Demodulator Module Functional Description