Application Note. Measuring distortion and Un-equalized MER

Transcription

1 Application Note Measuring distortion and Un-equalized MER The Verification Experts

2 Background Modern Cable Modems, Set-top-boxes and Cable Modem Termination Systems (CMTS) use advanced Adaptive Equalizer technology to compensate for complex in-channel frequency response impairments caused by micro-reflections, amplitude ripple and group delay occurring in the cable network. Rather than equalizing the entire upstream or downstream RF frequency spectrum, an adaptive equalizer adjusts its characteristics based on a single digitally modulated QAM carrier only as channel conditions change. This process maximizes or greatly improves the Modulation Error Ratio (MER) in the forward or reverse path. The adaptive equalizer is a digital FIR filter with coefficients that are changing (adapting) as the complex (amplitude and phase) in-channel frequency response of the signal under test changes. As with other equalizers, the complex frequency response of the adaptive equalizer at any given time is essentially opposite that of the signal under test. The ideal equalizer coefficients for the adaptive equalizer maximize MER by minimizing the impairments of the signal under test. If a change is detected in the impairments of the signal under test, the adaptive equalizer will quickly (usually within milliseconds) change its coefficients to maintain a complex frequency response that is equal and opposite to that of the signal under test. Equalized and Un-Equalized MER measurements Un-equalized MER is typically measured before the adaptive equalizer and equalized MER is measured after the adaptive equalizer but often this circuitry resides in the QAM demodulator and cannot be disabled. So while the adaptive equalizer performs a great job of improving MER of a QAM signal, it does makes troubleshooting marginal amplifiers, small ingress, CPD and related impairments a lot more difficult. To solve this problem, the CX180 adaptive equalizer can be turned off, allowing a technician to easily identify, sectionalize and locate these difficult linear distortion problems. Theory A QAM signal always contains a certain amount of distortion. There are many types of distortions, but they can be grouped into linear and non-linear distortions. Linear distortion is a distortion that is applied equally to all components of the signal, such as bandwidth filtering, reflection, etc. A filter with the inverse response can restore the original signal. The adaptive equalizer is such a filter and it removes the linear distortions of the incoming signal. Non-linear distortion is time dependent and signal dependent such as noise, compression, shape distortion, etc. No filter can restore the signal. QAM demodulators operate with a built-in equalizer so the calculated MER includes the effect of the equalizer. This is true for almost all demodulators. Only very sophisticated test equipment have the ability to turn off the equalizer. With the equalizer turned on, the MER reflects the nonlinear distortions of the signal, while linear distortions have been removed. In fact the adaptive mechanism of the equalizer strives to improve the MER until only (or almost only) the non-linear distortion remains. MER is the ratio of the signal power to the distortion power. Distortion power is the sum of the powers of many distinct distortion components, such as noise, interference, reflections, etc. Since the equalized MER only reflects non-linear distortions, the un-equalized MER can be determined by measuring the distortion power due to linear impairments and adding this power component to the non-linear distortion power in the MER equation. The equalizer operates by feeding into the main tap a controlled amount of the other tap signals. The filter coefficients are the controlling elements such that what is fed into the main tap is equal in amplitude and opposite in phase to the linear distortions, thus canceling them from the original signal. We can calculate the power fed into the main tap by summing the filter coefficients (in a linear mode, not in db). Consequently we can measure the power of the linear distortion of the incoming signal. 2

3 Measurement usefulness We have been trained to consider MER to be the comprehensive measurement of the quality of a QAM signal. It does give a good indication of the health of the signal, but it provides no information regarding linear impairments. The intent of splitting the impairments into linear and non-linear components is to provide more information that will allow the technician to identify and correct problems. Linear distortions are typically caused by filtering or reflections. Filter problems are quite often a diplexer or signal trap issue. While the equalizer compensates for linear distortions as much as possible, any remaining linear distortions after the equalizer will affect the MER. Non-linear distortion cannot be filtered out. In the distribution plant, the major contributor to nonlinear distortion is noise (thermal noise, analog/digital CTBCSO). At the head-end, non-linear distortions include distortions but this is not an issue since the introduction of all-digital QAM modulators. The following un-equalized MER values are considered minimum acceptable values for proper operation in CATV plants: QPSK: 18dB 16-QAM: 24dB 64-QAM: 27dB 256-QAM: 31dB The following un-equalized MER ranges are typically where problems become very apparent: QPSK: 10-13dB 16-QAM: 17-20dB 64-QAM: 22-24dB 256-QAM: 28-30dB Un-equalized MER reflects both linear and non-linear distortions. Comparing equalized and un-equalized MER indicates the relative effects of both. 3

4 In the field Un-equalized MER is a measurement support in VeEX field products CX180 and CX380. It is standard feature and is part of the QAM Analysis package. While useful for the forward path, un-equalized MER becomes even more important for the return path with DOCSIS 3.0 deployments using 64-QAM and 128-QAM signals. The VeEX CX180 and CX380 can be used in conjunction with the CX120 equipped with its upstream generator. The VeEX upstream generator can generate various signals including QAM-64, QAM-128 and QAM-256 with FEC data allowing the receiving unit to measure not only MER but also BER. Coax CX180 Fiber Optic Network CX120 4

5 Related products VePAL CX120 Upstream Generation of CW, QPSK, QAM16/64/128 with FEC Ingress (Forward/Reverse) Check for fast troubleshooting VePAL CX180 Upstream Analysis of QAM16/64/128 with MER and Constellation Advanced Spectrum Analysis Check for fast troubleshooting Optional DOCSIS Testing Optional TDR Testing VePAL CX350 Advanced single-channel SLM measurements with Min/Max thresholds High definition DVB-C carriers and DOCSIS 3.0 cable modem emulation/ measurement Optional Upstream Generation of QAM16/64/128, CW, and QPSK signals Optional TDR Testing VePAL CX380 Advanced Spectrum Analysis Advanced QAM Measurements Optional Upstream Generation of CW, QPSK, QAM16/64/128 with FEC Optional DOCSIS Testing Check for fast troubleshooting The Verification Experts VeEX, Inc Martin Ave., Suite G Santa Clara, CA 95050, USA Tel: Fax: customers@veexinc.com 2009 VeEX Inc. All rights reserved. VeEX is a registered trademark of VeEX Inc. The information contained in this document is accurate. However, we reserve the right to change any contents at any time without notice. We accept no responsibility for any errors or omissions. In case of discrepancy, the web version takes precedence over any printed literature. D P A /09