CMX7163 QAM Modem
CMX7163
Tx PRBS
In Tx mode, a fixed PRBS (pseudo random bit sequence) or a repeated preamble transmission is provided
and may be used for test and alignment. A 511 bit PRBS conforming to ITU-T O.153 (Paragraph 2.1) is
used to generate the PRBS.
The output created by transmitting a PRBS using 16-QAM is shown in Figure 17. The 16 constellation
points are just visible on the plot.
Figure 17 Transmit Constellation
Rx Constellation
A test mode to examine the Rx constellation diagram is also provided, this utilises the IOUTPUTP/N and
QOUTPUTP/N pins to produce a diagnostic signal where the RRC filtered I/Q signals are output. This
produces a two-dimensional constellation diagram which may be displayed on an oscilloscope in X-Y
mode. Note that best results are often obtained with an analogue oscilloscope.
Figure 18 Constellation
Diagram – no frequency or
phase error
Figure 19 Constellation
Diagram – phase error
Figure 20 Constellation
Diagram –frequency error
As shown in the third plot, if there is any frequency error between transmitting and receiving CMX7163
devices then the diagram will spin and be difficult to interpret. Therefore other diagnostic modes are
provided as described below.
Any of the GPIO signals can be configured to produce a pulse train at the nominal symbol rate of the
receiving CMX7163 to aid triggering whilst viewing the constellation diagram (I Output or Q Output alone
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