Promax US TV EXPLORER II, Руководство пользователя

Страница: 72 / 268

USER’S MANUAL. US TV EXPLORER II
Page 64 01/2008
0 10 20 30 40
NM: 8.4 dB
FREQ:
CH:
C/N:
POWER:
MER:
SER:
VBER:
»
>21.3
55.8
19.5
dB
dBµV
dB MHz
kHz
57.00
-3
A02
1.0E-5
1.0E-8
MPEG-2
Figure 44 .- Screen of MER measurement of ATSC (8-VSB) signals.
First of all, you will see the modulation error ratio measurement: MER .
Following, it appears the Noise Margin ( NM ) measurement (in the figure value
8.4 dB). It indicates a safety available margin according to the MER level measured that
allows signal degradation until arriving to the TOV ( Threshold of Visibility ) or QEF
(quasi error free) limit value.
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.