Nautel NX25, Operation And Maintenance Manual

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NX300 Operations and Maintenance Manual Operating the transmitter
Page 2-36 Issue 3.0 2012-11-01
Figure 2.22: Signal Constellation
Signal constellation.
See Figure 2.22 . In IBOC mode only, 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. You can select sub-
carriers by pressing on the carrier representation in the
SubCarrier Group MER Spectrum
chart, or
by selecting the desired subcarrier from the
SubCarrier Group Details
list. Typically, the dots will be
grouped together around the ideal data points.
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. The MER out of the transmitter is typically better
than 20 dB, but can be significantly affected by external influences such as antenna impedance and
bandwidth.
NOTE:
When subcarriers are turned off in the Exgine settings, they will still appear in this screen, but
the MER will be very high (i.e., 0 to 10 dB).
Use the up or down button to maximize (if it was minimized) or minimize (if it was maximized) the
panel size.