Atec Tektronix-DMA120-DMA121 User Manual
Page 2
page 2
Qualify System Performance
The built-in digital demodu-
lator makes possible in-ser-
vice measurements of:
• Modulation Error Ratio
(MER)
• Error Vector Magnitude
(EVM)
• Estimated Noise Margin
• BER before Reed-Solomon
(R-S) decoder
• Estimated BER after R-S
decoder with system avail-
ability statistics
Modulation Error Ratio
(MER): ETSI ETR290 indi-
cates that MER is the best
overall “figure of merit” mea-
surement to determine QAM
signal quality. Although bit
error rate (BER) has been
widely addressed as an
important digital transmis-
sion “figure of merit” mea-
surement, MER provides a
much earlier indication of
transmission impairments.
MER indicates the ratio of
average total signal power in
the ideal constellation to
average error power in the
constellation as received by
the DMA120 Series. The mea-
surement includes error
power due to any impair-
ment. If the only impairment
in the test channel spectrum
is noise, MER is equal to sig-
nal-to-noise. Technicians will
feel familiarity with MER
because results are expressed
in dB, similar to analog trans-
mission carrier-to-noise or
signal-to-noise measure-
ments. The DMA120 Series
measurement screen (see
Figure 1) shows MER vs.
time, providing trend infor-
mation.
Error Vector Magnitude
(EVM): EVM is an alternative
“figure of merit” measure-
ment. It has been a perfor-
mance measurement for digi-
tal communication systems
in the past and provides a
means of system comparabil-
ity for some engineers. The
same distortion elements are
measured as in MER. How-
ever, the calculation of error
is different and is expressed
as a percentage (%) of the
maximum voltage in the con-
stellation (at sampling times)
as received by the DMA120
Series.
Estimated Noise Margin: The
legacy of rf broadband system
maintenance has been based
on measurements in the fre-
quency domain. Technicians
are conditioned to thinking of
system “headroom” in terms
of dB carrier-to-noise or dBc
for CSO, CTB, or cross modu-
lation. Estimated Noise Mar-
gin indicates the “headroom”
for digital channels and
results are reported in dB,
similar to legacy analog mea-
surements. Essentially, simu-
lated gaussian noise is added
to the input signal until a
critical pre-FEC BER of 10
–4
is measured. The added, sim-
ulated noise equals the Esti-
mated Noise Margin which
answers the question, “how
many dB until subscribers are
receiving impaired digital
services?”
Bit Error Rate (BER) before
Reed-Solomon (R-S) decod-
ing: BER is an important mea-
surement to document sys-
tem performance. The typical
system operating goal is to
achieve a BER of 10
–9
. Ser-
vice impairment will typi-
cally be observed at bit error
rates greater than 10
–4
. The
BER measurement is the aver-
age ratio of bit errors to total
bits received in a specified
time period. The DMA120
Series BER measurement may
be set for any period from 1
to 60 minutes. Results from
successive periods can be
compared to show a
24-period trend.
Figure 1. MER and Estimated Noise Margin screen.
Figure 2. BER screen.