Understanding and Troubleshooting Linear Distortions: Micro-reflections, Amplitude Ripple/Tilt and Group Delay RON HRANAC 1
A Clean Upstream: Or Is It? Graphic courtesy of Sunrise Telecom 2
Transmission Line Theory 101 Source Transmission Medium Load Z S = Z T = Z L Source Transmission Medium Load Source Z S? Z T? Z L Transmission Medium Load Z S? Z T? Z L 3
Impedance Mismatches Source (7 db return loss) +31 dbmv at time T 0 +22 dbmv Transmission Medium (1 db attenuation) Load (7 db return loss) +30 dbmv at time T 1 +23 dbmv +15 dbmv +14 dbmv (-16 dbc) at time T 2 100 ft 23 20 Amplitude (dbmv) +30 +25 +20 +15 +10 +5 +30 dbmv +14 dbmv 234 ns Incident signal (T 1 ) Echo (T 2 ) Time (T) 4
Micro-reflections The Big Picture Amplitude (dbmv) +30 +25 +20 +15 +10 +5 +30 dbmv 234 ns +14 dbmv (-16 dbc) Incident signal (T 1 ) Echo (T 2 ) Amplitude (dbmv). Frequency (F). Null (+28.5 dbmv) Peak (+31.28 dbmv) Amplitude ~4.27 MHz Time (T) Time (T) Echo calculation courtesy of Holtzman, Inc. 5
Micro-reflections Causes: Damaged or missing end-of-line terminators Damaged or missing chassis terminators on directional coupler, splitter, or multipleoutput amplifier unused ports Loose center conductor seizure screws Unused tap ports not terminated; this is especially critical on low value taps, but all unused tap ports should be terminated with 75-ohm terminations (locking terminators without resistors or stingers do not terminate the tap port) Poor isolation in splitters, taps and directional couplers Unused customer premises splitter and directional coupler ports not terminated Use of so-called self-terminating taps at feeder ends-of-line; these are the equivalent of splitters, and do not terminate the feeder cable unless all tap ports are terminated Kinked or damaged cable (includes cracked cable, which causes a reflection and ingress) Defective or damaged actives or passives (water-damaged, water-filled, cold solder joint, corrosion, loose circuit board screws, etc.) Cable-ready TVs and VCRs connected directly to the drop (return loss on most cableready devices is poor) Some traps and filters have been found to have poor return loss in the upstream, especially those used for data-only service 6
Amplitude Ripple/Tilt Amplitude ripple and tilt are known in cable industry vernacular as frequency response, and can refer to in-channel frequency response or the frequency response across a specified RF bandwidth such as 5-42 MHz The causes include gremlins such as improper network alignment and impedance mismatches (micro-reflections!) Amplitude tilt Combination of amplitude ripple and tilt Amplitude tilt, slight amount of ripple Graphics courtesy of Acterna and Sunrise Telecom 7
Amplitude Ripple/Tilt Adaptive equalization This 6.4 MHz bandwidth A-TDMA digitally modulated signal exhibits severe in-channel amplitude tilt at the CMTS upstream input Adaptive equalization (preequalization) in the cable modem is able to compensate for nearly all of the amplitude tilt 8
Group Delay Group delay is said to exist when signals at some frequencies take longer to travel through a circuit, amplifier or network than signals at other frequencies Group delay, like other linear impairments, causes intersymbol interference, which degrades MER Graphic courtesy of Holtzman, Inc. 9
Linear Distortions in the Real World In this example, an echo at ~485 ns causes visible amplitude ripple across the 5-42 MHz spectrum Group delay ripple also is present Echo Amplitude ripple Group delay ripple Graphic courtesy of Holtzman, Inc. 10
Linear Distortions in the Real World Here s another example: An approx. -33 dbc echo at just over 1 µs This echo meets the DOCSIS upstream - 30 dbc at >1.0 µsec parameter Here, too, the echo is sufficient to cause some amplitude and group delay ripple Graphic courtesy of Sunrise Telecom 11
A Clean Upstream: Or Is It? Remember the upstream slide at the beginning of this presentation? Here s why 16-QAM wouldn t work Graphic courtesy of Sunrise Telecom 12
A Clean Upstream: Or Is It? This upstream constellation shows a not-sogood 16-QAM signal Unequalized MER is 21.3 db, close to the failure threshold for 16-QAM Graphic courtesy of Sunrise Telecom 13
A Clean Upstream From a linear distortion perspective, this is what a relatively unimpaired upstream looks like Graphics courtesy of Sunrise Telecom 14
Wrapping Up Linear distortions are real problems in cable networks, and can seriously affect downstream and upstream data transmission Among the tools available to troubleshoot linear distortions are Forward and reverse sweep, set to the maximum supported resolution Adaptive equalization (DOCSIS 1.1 and 2.0 modems) CMTS tools such as per-modem SNR (MER), FEC error information Avoid upstream frequencies above about 35 MHz to minimize diplex filter-related group delay Use of specialized test equipment to characterize and troubleshoot micro-reflections, amplitude and group delay ripple An understanding of linear distortions is critical to achieving the reliability necessary for new services being deployed on today s cable networks 15