Atec DMA120, DMA121 User Manual

Affordable, portable, ready for
cable system technicians: digital
channel 64 and 256QAM
transmission performance
verification measurements

The DMA120 Series Digital
Modulation Analyzers pro­vide answers to nagging ques­tions about how to test and
verify performance of your
digital plant. Quantification
of digital transmission perfor­mance – at any point in your
system – will greatly enhance
technical management and
decision making. The
DMA120 provides analysis of
ITU-T-J.83, Annex B 64 and
256QAM and the DMA121
verifies 64 and 256QAM for­matted per DVB-C.

The DMA120 Series provides
the intelligence cable TV
technicians need to effi­ciently install and maintain
HFC distribution plant using
64 or 256QAM transmission.
Measurement results provide
a clear picture of system per­formance, minimizing the
need to re-visit an installa­tion site or make additional
service calls. The DMA120
Series field tools are housed
in a rugged, weather resistant
package and are powered by
an easy-to-change NiMh bat­tery. Standard accessories
include a protective soft case,
mains power supply, and
user’s manual.

Copyright © 1999 Tektronix, Inc. All rights reserved.

DMA120, DMA121
Digital Modulation
Analyzers

64 and 256QAM Analysis
In-service Digital Channel

Performance Verification
Measurements at an Affordable
Price

Measurements Compliant with DVB
(ETSI) Standard ETR290

Troubleshooting Aided by
Constellation, Spectrum, and
Equalization Display Modes

Most Measurements Require Only
Two Keystrokes

Pass/fail Indication Reduces
Training Requirements for
Technicians New to Digital Signal
Measurements

PC Card (PCMCIA) Slot for Easy
Memory Expansion

Small and Light Weight
Easy-change Battery

page 2

Qualify System Performance

The built-in digital demodu­lator makes possible in-ser­vice 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 avail­ability statistics

Modulation Error Ratio
(MER): ETSI ETR290 indi-

cates that MER is the best
overall “figure of merit” mea­surement to determine QAM
signal quality. Although bit
error rate (BER) has been
widely addressed as an
important digital transmis­sion “figure of merit” mea­surement, 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 mea­surement includes error
power due to any impair­ment. If the only impairment
in the test channel spectrum
is noise, MER is equal to sig­nal-to-noise. Technicians will
feel familiarity with MER
because results are expressed
in dB, similar to analog trans­mission carrier-to-noise or
signal-to-noise measure­ments. The DMA120 Series
measurement screen (see
Figure 1) shows MER vs.
time, providing trend infor­mation.

Error Vector Magnitude
(EVM): EVM is an alternative

“figure of merit” measure­ment. It has been a perfor­mance measurement for digi­tal communication systems
in the past and provides a
means of system comparabil­ity for some engineers. The
same distortion elements are
measured as in MER. How­ever, the calculation of error
is different and is expressed
as a percentage (%) of the
maximum voltage in the con­stellation (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 fre­quency domain. Technicians
are conditioned to thinking of
system “headroom” in terms
of dB carrier-to-noise or dBc
for CSO, CTB, or cross modu­lation. Estimated Noise Mar­gin indicates the “headroom”
for digital channels and
results are reported in dB,
similar to legacy analog mea­surements. Essentially, simu­lated gaussian noise is added
to the input signal until a
critical pre-FEC BER of 10

–4

is measured. The added, sim­ulated noise equals the Esti­mated 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) decod­ing: BER is an important mea-

surement to document sys­tem performance. The typical
system operating goal is to
achieve a BER of 10–9. Ser­vice impairment will typi­cally be observed at bit error
rates greater than 10–4. The
BER measurement is the aver­age 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.

page 3

Estimated BER after R-S
decoding: BER after R-S

decoding represents the ser­vice level that the cable sys­tem is providing to the sub­scriber’s TV receiver or com­puter. 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 num­ber 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 mea­surements.

Maintain and Troubleshoot Your
Digital Video Plant

When performance quality
measurements indicate trans­mission problems, the
DMA120 Series can provide
assistance to find out what’s
wrong. Additional measure­ments 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 integrat­ing 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, provid­ing a convenient display from
which to make gain or attenu­ation adjustments or judge
other variables that can
impact signal level.

Adjacent Channel
Levels: Potential interference

to adjacent channels can be
verified using this measure­ment. 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 mea­surement results screen
includes a bar graph compar­ing the test channel and adja­cent 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 flat­ness, and relative signal lev­els can be visually confirmed
(see Figure 3).

Constellation Display
Mode: Digital modulation

quality can be visually esti­mated by viewing the con­stellation of the transmitted
signal (see Figure 4). Distri­bution impairments can be
identified including noise
and coherent interferers or
modulator impairments such
as I/Q imbalance or quadra­ture 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 bound­aries 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.