Nautel VS1, Operation And Maintenance Manual

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VS1 Operations and Maintenance Manual Operating the transmitter
Page 2-44 Issue 3.1 2013-10-01
Figure 2.31: Signal Constellation
Signal constellation
See Figure 2.31 . 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.
Use the scroll bar to select a higher or lower sub-carrier for viewing. Some sub-carriers are for timing
and synchronization. Others are modulated with data/content. Use the maximize or minimize
buttons as required.