17
WavePlex ™
Digital Demodulator
User’s Manual
SX061
1 Mbit/s DQPSK/DBPSK
Designing a Link Using the SX061 and the
SX043/42
Continued
4. Setup the AGC Loop
The AGC loop can be closed and tested now. This has
to be tested using an unmodulated (no spreading/
despreading) carrier.
Note: COMP_SEL register has to be set to 0.
5. Setup the Internal IF Frequency
The IF frequency is selected by programming the
FOFFSET register. This will make the NCO run at the
IF frequency. For a description on how to choose a
value for this register see the NCO section.
To check if there is correlation between the incoming
signal and the internal IF, the RSSI outputs can be
monitored.
6. Setup the RSSI Outputs
By choosing the values of ID_GAIN and PWM_GAIN
the RSSI signal can be scaled. This has to be done in
such a way that the output of the RSSI signal is about
half the output scale (_ * Vdd). The effect on the RSSI
outputs caused by changing the values of ID_GAIN
and
PWM_GAIN
is
as
follows:
Increasing
the
PWM_GAIN register by one, will decrease the RSSI
outputs by a factor two. Increasing the ID_GAIN
register by one, will decrease the RSSI outputs by a
factor four. This is because the I&D outputs are
squared, to deduce an energy level from them. So
dividing them by two results in an energy level divided
by 4. Do not increase the I&D gain value too much
when increasing the PWM_GAIN value is also
possible. This is because increasing the I&D gain value
is like throwing away information (the LSB bits of X and
Y). These bits will also be lost for data demodulation
and carrier tracking.
Note: While the TRACKON signal is low, there is no difference
between RSSI1 and RSSI2.
Choosing another FOFFSET value should result in a
significant decrease in the RSSI output, as indicated by
the I&D transfer graph. This graph shows that when the
frequency of the input signal is equal to Foffset, the
RSSI output is maximal. When the difference between
the input signal and Foffset is 0.5 * Fsymbol, then the
output is minimal. On the top of the first side lobe,
where FIF-Foffset= _ * FSymbol the output is about 5% of
the nominal output.
This is also a check for a correct setup of the internal
NCO.
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