Nautel VSHD Exciter, Инструкция по эксплуатации

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VSHD Exciter Technical Instruction Manual
Page 3-5
Section 3 Operation Issue 2.0
3.1.1.3 SIGNAL CONSTELLATION
See Figure 3-5. 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.
Figure 3-5: Signal Constellation