Nautel NV7.5, Operation And Maintenance Manual

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NV10/NV7.5 Operations and Maintenance Manual Operating the transmitter
Issue 3.4 2016-08-03 Page 2-41
Figure 2.27: Signal Constellation
Signal constellation
See Figure 2.27 . The exciter constantly measures the transmitter signal and performs basic
demodulation of the digital carriers. The
Signal Constellation
panel displays the phase and
amplitude of the symbols being modulated within an OFDM sub-carrier as dots on a cartesian
graph. There are separate screens for each sub-carrier. Typically, the dots will be grouped
together around the ideal data points. When the transmitter is on, the signal constellation
display is representative of the transmitter output. When the transmitter is off, the display is
representative of the forward path that will be transmitted.
Sub-carrier group information is shown in the lower, right section of the screen. Displayed
information includes the sub-carrier group name, the bandwidth that the selected carrier group
occupies and the modulation error ratio (MER) for the selected carrier group. MER quantifies
the
performance of the transmitted digital signal as the ratio between the RMS power of the ideal
signal
and the RMS power of the received signal’s error vector. A higher MER value is characteristic
of a
smaller error, and therefore a higher quality signal. There is a NRSC minimum requirement of
a MER value of no less than 14 dB.