Constellation Diagram - Promax US TV EXPLORER II Manual De Uso

USER'S MANUAL. US TV EXPLORER
In the case of a DVB-S2 signal (QPSK/8PSK) instead of the Noise Margin
appears the measure of the Link Margin (LM); in the previous figure with a value of
2.3 dB. The LM is equivalent to the NM and indicates the distance to the QEF
(generally defined as one lost packet per hour). The LM is measured in dB and its value
is equal to the safety margin that separates us from the QEF. As bigger LM better signal
quality. An LM with a negative value means that there is no signal reception or errors
are beginning to display clearly in the video or the audio. An LM equal to 0 (zero)
displays a service and occasionally some artefacts can be observed.
Analogue and digital carriers are very different in terms of signal contents and
power distribution over the channel. They, therefore, need to be measured differently.
The modulation error ratio (MER), used in digital systems is similar to the Signal/Noise
(S/N) ratio in analogue systems.
The MER represents the relation between the average power of ATSC signal and
the average power of noise present in the constellation of the signals.
By example, 8-VSB demodulators require a MER greater than 15 dB to work.
Though it is preferable to have at least a 3 or 4 dB margin to compensate for any
possible degradation of the system. While QAM Annex-B 64 demodulators require a
MER greater than 21 dB or QAM Annex-B 256 greater than 28 dB with margins of at
least 3 dB. Normally, the maximum MER value seen in portable analyzers is of
approximately 34 dB.
Finally it is shown a status line, which displays information about the detected
signal.
5.15

Constellation Diagram

The constellation diagram is a graphic representation, called I-Q, of the digital
symbols received over a period of time.
There are different types of constellation diagrams for the different modulation
modes. With the US TV EXPLORER
and DVB-S2 e ITU-T 1.83/B signals.
In the case of an ideal transmission channel, free of noise and interferences, all
symbols are recognized by the demodulator without mistakes. In this case, they are
represented in the constellation diagram as well defined points hitting in the same area
forming a clear dot.
Noise and impairments cause the demodulator to not always read the symbols
correctly. In this case the hits disperse and create different shapes that at the end will
allow to determine at a glance the type of noise in the signal.
11/2013
II
II it is possible to display constellations for DVB-S
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