Table of Contents
Advertisement
AD INSTRUMENTS
S7200 TV Signal Analyzer Operation Manual
In average power measurement interface, the screen display 8VSB signal average
power, MER, Pre-BER and Post-BER. The average power use the histogram display.
8VSB constellation diagram include 8 vertical lines. Every line is compoised by
many constellation points. Every constellation point's horizontal coordinate (or
data real part) carry information, vertical coordinate (or data imaginary part)
doesn't carry information. The thinner the vertical line, the better the signal. The
thicker the vertical line, the worse the signal. The constellation daigram can be
used to quick check the signal quality. Bowed lines indicate amplitude error such
as clipping or nonlinear expansion. (AM-AM distortion) S-shaped lines indicate
phase error (AM-PM distortion). Lines frayed at edges indicate phase noise
problems.
Figure 6-27 8VSB Constellation
Another way to determine the signal quality is with the eye diagram. Look for the
seven open eyes in the eye diagram. If some eyes are not fully open indicates that
the signal has some error, and the 8 vertical bars are not as compact as they could
be. A perfectly compacted signal would have straight vertical line on the 8VSB
constellation and open eyes in the eye diagram. The Figure 6-29 shows decreased
signal quality, eye diagram gradually closed.
145
Advertisement
Table of Contents
