Atec Tektronix-DMA120-DMA121 - Manual (Page 3)

page 3
Estimated BER after R-S
decoding: BER after R-S
decoding represents the ser-
vice level that the cable sys-
tem is providing to the sub-
scriber’s TV receiver or com-
puter. The R-S decoder is
typically able to correct
errors up to an input BER of
approximately 10
–4 . Beyond
this value, uncorrected errors
pass through the decoder
– this is the value reported by
Estimated BER (see Figure 2).
System availability statistics
are also reported by the
DMA120 Series, including:
• Errored seconds (number of
seconds that include an
errored block)
• Severely errored seconds
(one-second periods with
greater than specified num-
ber of errored blocks)
• Severely disturbed periods
(duration of sync loss)
• System unavailability time
(time period containing at
least 10 consecutive
severely errored seconds)
System availability statistics
are very useful for tracking
intermittent impairments that
may not be observable based
on MER or average BER mea-
surements.
Maintain and Troubleshoot Your
Digital Video Plant
When performance quality
measurements indicate trans-
mission problems, the
DMA120 Series can provide
assistance to find out what’s
wrong. Additional measure-
ments and display modes
that can help the technician
include:
• Channel Average Power
(Signal Level)
• Adjacent Channel Levels
• Spectrum Display Mode
• Constellation Display Mode
with zoom capability
• Adaptive Equalizer Display
Mode
Digital Channel Average
Power: RF signal level mea-
surement is made by integrat-
ing all the channel power
through a channel bandwidth
IF filter. A user-settable offset
is available to accommodate
probe loss. The Signal Level
measurement screen plots
signal levels vs. time, provid-
ing a convenient display from
which to make gain or attenu-
ation adjustments or judge
other variables that can
impact signal level.
Adjacent Channel
Levels: Potential interference
to adjacent channels can be
verified using this measure-
ment. The Adjacent Channel
Levels display compares the
test channel average power to
the average power in each of
the two higher and two lower
adjacent channels. The mea-
surement results screen
includes a bar graph compar-
ing the test channel and adja-
cent channels power. This
measurement may be used to
check flatness across five
channels or verify spectral
purity of a QAM modulator.
Spectrum Display Mode: The
channel or system spectrum
can be viewed in this mode
– channel symmetry and flat-
ness, and relative signal lev-
els can be visually confirmed
(see Figure 3).
Constellation Display
Mode: Digital modulation
quality can be visually esti-
mated by viewing the con-
stellation of the transmitted
signal (see Figure 4). Distri-
bution impairments can be
identified including noise
and coherent interferers or
modulator impairments such
as I/Q imbalance or quadra-
ture error. The DMA120
Series also includes two
zoom modes – a constellation
quadrant view and a single
constellation box can be
selected to closely examine
the distribution of symbol
landings. Decision bound-
aries are displayed along
with adjacent boxes to make
it easy to see the symbol
landing patterns (see
Figure 5).
Figure 3. Spectrum mode.
Figure 4. Constellation display mode.