L64704 LSI Logic Corporation, L64704 Datasheet - Page 195

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

B.2.4
QPSK
Demodulator
Debugging
Summary
The following steps summarize how to debug the QPSK Demodulator
portion of a system that includes the L64704.
Step 1. Ensure that the AGC loop functions properly.
Step 2. Ensure that the clock loop is locked.
Step 3. Set the CAR_SW and CAR_OPEN bits in the Carrier Loop
Step 4. If the carrier always goes to the rail, then change the
L64704 QPSK Demodulator Debugging Tips
–
If the carrier always goes to the rail, then change the
CAR_SWP_SWAP and CAR_VCO_SWAP bits in the Carrier Loop
Configuration register. These bits control the sweep direction and the
polarity of the CAR_VCOxN/P outputs. If changing polarity does not
help, disconnect the op-amp’s output and drive the VCO with a
power supply to see whether changing the voltage controls the
frequency.
If the prescaler output is at a proper frequency and the op-amp’s out-
put is not at the rail, look at the eye pattern to see where the data is
being sampled. A good test is to trigger the ADC’s I or Q inputs
against the ADC’s clock (see
must sample the data at the maximum eye opening.
Set the CAR_SW and CAR_OPEN bits in the Carrier Loop Con-
figuration register to b01 and see if the carrier locks. The
microcontroller should monitor the content of the
STATUS register (Group 3, APR 9). If the CAR_LC and
CAR_LCF bits both = 1, then clear bits 0 and 1 of the
CAR_CONFIG register to stop the sweep. If the sweep rate is
fast, you may not be able to turn the sweep off fast enough
through manual intervention; the microcontroller will have to do
this in real time.
Configuration register to b11. This setting forces sweep to
begin and disables the AFC.
CAR_SWP_SWAP and CAR_VCO_SWAP bits in the Carrier
Change the Carrier Upper and Lower Sweep Limits registers
(Group 4, APR 29:32) and watch the carrier prescaler with a
spectrum analyzer to see how the programmed values con-
trol the sweep range.
Change the value in the Carrier Sweep Rate register (Group
4, APR 28) to ensure that it is controlling the sweep rate.
Figure 4.1
on
page
4-2). The clock
B-7