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.

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

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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.

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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.

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