upstream rf troubleshooting

7/30/2019 Upstream RF Troubleshooting

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1112003, Cisco Systems, Inc. All rights reserved.Upstream RFTroubleshooting


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Upstream RF Troubleshooting

Ron Hranac

Technical Leader


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CMTS Configuration

Check this firstincorrect cable modemtermination system (CMTS) configurationis a common problem!


4/664442003, Cisco Systems, Inc. All rights reserved.Upstream RFTroubleshooting

Upstream Challenges

Most cable systems use a sub-split bandplan

50-860 MHz downstream, 5-42 MHz upstream

Problems with sub-split in two-way

networks:Upstream noise funneling

Prevalence of manmade noise in upstreamfrequency spectrum

Lack of upstream reference signals

Difficult to locate problems



Check the Upstream at the CMTS

Connect a spectrum

analyzer to the

upstream test points



Upstream Integrity

Verify that the upstream digitallymodulated carrier amplitude at the input tothe CMTS upstream port is within spec

A typical value is 0 dBmV, but this may vary

depending on the CMTS manufacturers specsand CMTS configuration

Check the upstream carrier-to-noise,

carrier-to-ingress, and carrier-to-interference ratios

DOCSIS assumes a minimum of 25 dB for all

three parameters



Digitally Modulated Carrier Amplitude

The zero-span method isthe easiest way to obtain

an accurate amplitudemeasurement

Because of the burstynature of upstream

digitally modulatedcarriers, its diff icult tomeasure average powerlevel



Upstream Spectrum

Does the upstreamlook like this?

or like this?


9/669992003, Cisco Systems, Inc. All rights reserved.

Upstream RFTroubleshooting

Check the Upstream at the CMTS

Specialty test

equipment may beused to evaluate

upstream

constellations




Upstream Constellations

Ideal QPSK and 16-QAM constellations

Graphics courtesy of Filtronic Sigtek, Inc.





Poor carrier-to-noise ratio



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CW carrier interference



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Upstream RF Impairments

Stationary Impairments

Thermal noise

Intermodulation distortion

Frequency response

Transit delay

Group delay


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Transient Impairments

RF ingress

Impulse noise

Signal clipping


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Multiplicative Impairments

Transient hum modulation

Intermittent connections


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Thermal Noise

Characteristic of all active components:

Optoelectronics

Upstream amplifiers

In-home devices

Improper network alignment or defectiveequipment can cause high levels ofthermal noiseas can improper upstreamcombiningwhich will degrade carrier-to-noise ratio


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Thermal Noise

Good carrier-to-noiseratio (~50 dB)

Poor carrier-to-noise

ratio (~12 to 15 dB)


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Intermodulation Distortion

Second and third order distortions mostprevalent

Active devices Passive components: common path

distortion, passive device intermodulation


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Intermodulation Distortion

An example ofcommon path distortion

Note large 2nd order beats spaced every 6 MHz,

and smaller 3rd order beats +/-1.25 MHz from2nd order beats

2nd order beats

3rd order beats


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Frequency Response

Amplifier alignment

Input and output levels

Proper pads and equalizers

Sweep versus multiple carriers

Alignment-related problems

Frequency response problems can cause group delay

errorsMisalignment can cause increase in noise anddistortions

Microreflections


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Frequency Response

Defective coaxial cable caused frequency

response problem


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Microreflections

Microreflectionsalso called reflections or echoesare caused by impedance mismatches

In the real world of cable networks, impedance can atbest be considered nominal

Impedance mismatches are everywhere: connectors,amplifiers inputs and outputs, passive device inputsand outputs, and even the cable itself

Upstream cable attenuation is lower than

downstream cable attenuation, so upstreammicroreflections tend to be worse

Anywhere an impedance mismatch exists, some of

the incident energy is reflected back toward thesource


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Microreflections

The reflected and incident energy interact toproduce standing waves, which manifestthemselves as the standing wave amplituderipple one sometimes sees in sweep receiverdisplays

16-QAM is affected by microreflections to a muchgreater degree than QPSK is

Microreflections and group delay may becompensated for using adaptive equalization, afeature available in DOCSIS 1.1 and 2.0 cablemodems

Adaptive equalization is not supported by most

DOCSIS 1.0 modems


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Microreflections

Damaged or missing end-of-line terminators

Damaged or missing chassis terminators ondirectional coupler, splitter, or multiple-outputamplifier unused ports

Loose center conductor seizure screws

Unused tap ports not terminatedthis is

especially critical on low value taps Unused drop passive ports not terminated

Use of so-called self-terminating taps at feederends-of-line

Causes:


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Microreflections

Kinked or damaged cable (includes cracked

cable, which causes a reflection and ingress)

Defective or damaged actives or passives (water-damaged, water-fi lled, cold solder joint,

corrosion, loose circuit board screws, etc.)

Cable-ready TVs and VCRs connected directly tothe drop (return loss on most cable-ready

devices is poor)

Some traps and fi lters have been found to havepoor return loss in the upstream, especially

those used for data-only service

Causes:


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Microreflections

In this example,an approx. -23dBc echo at~720 ns causesvisible

amplituderipple acrossthe 5-40 MHzspectrum

Group delayripple also ispresent

Echo

Ampl itude ripple

Group delay ripple

Courtesy of Holtzman, Inc.


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Microreflections

Heres anotherexample: An

approx. -33 dBcecho at just over 1sec

This echo meets

the DOCSISupstream -30 dBcat >1.0 secparameter

Here, too, theecho is sufficientto cause someamplitude andgroup delay ripple


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Transit Delay

Electromagnetic signals travel at thespeed of light

In free space the speed of light is 299,792,458meters/second

In CATV coaxial cable, it s about 87% of thefree space value

In optical fiber, its about 67% of the free space

value

RF and optical signals take a finite amount oftime to travel through a CATV network


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Transit Delay (contd)

Signals traveling one waysay, from the subscriberto the headendthrough 1 km of coax and 18 km of

fiber: about 95 microseconds (sec) transit delay The DOCSIS transit delay specification is


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Group Delay

From the IEEE Standard Dictionary of

Electrical and Electronics Terms:

Group delay is the derivative of radian phasewith respect to radian frequency. It is equal tothe phase delay for an ideal non-dispersivedelay device, but may differ greatly in actualdevices where there is a ripple in the phaseversus frequency characteristic.


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Group Delay (contd)

Group delay is defined in units of time,

typically nanoseconds (ns)

In a system, network or component with

no group delay, all frequencies aretransmitted through the system, networkor component with equal time delay.

Frequency response problems in a CATVnetwork will cause group delay problems.


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Group Delay (contd)

If a cable networks group delay exceeds acertain amount, data transmission and biterror rate may be affected.

As long as group delay remains below adefined thresholdDOCSIS specifies 200

nanoseconds/MHz in the upstreamgroupdelay-related BER shouldnt be a problem.


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Group Delay vs. MER

DOCSIS 200 ns/MHZ

QPSK16-QAM

QPSK typicallyrequires a

minimum MER of10~13 dB,depending on

CMTS make/model 16-QAM typically

requires a

minimum MER of17~20 dB,depending onCMTS make/model

Courtesy of Holtzman, Inc.


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Group Delay (contd)

Upstream

group delaymeasurementsrequirespecializedequipment

When obvious

problems havebeen ruled out,check group

delay


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Group Delay

Specialized

test equipmentcan be used tocharacterizeupstream in-

channelperformance

In thisexample, in-

channel groupdelay ripple isabout 60 ns

Courtesy of Sunrise Telecom


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RF Ingress

Upstream spectrum is shared with over-the-air

usersShort-wave broadcasts

Citizens band (CB ) radio

Amateur ( ham ) radio

Ship and aeronautical communications

Government communications RF signals can enter network through cable

shielding defect


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Upstream Over-The-Air Spectrum, 5-30 MHz

Source: NTIA (http://www.ntia.doc.gov/osmhome/allochrt.pdf)


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RF Ingress

CB radio operator had installed his own

cable outlets


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Impulse Noise

Most upstream data transmission errorscaused by bursts of impulse noise

Fast risetime, short duration (


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Impulse Noise

Impulse noise from arc welder in

machine shop

Si l Cli i


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Signal Clipping

RF ingress and impulse noise may cause

signal clipping Excessive signals from in-home devices

such as pay-per-view converters also maycause signal clipping

Clipping (compression) occurs in

upstream amplifiers and fiber opticsequipment

Upstream lasers most susceptible

Si l Cli i ( td)


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Signal Clipping (contd)

Most energy that causes compression isin 5 MHz to 15 MHz range

Signals at all other frequencies are

affected by cross-compressionCross-compression affects all upstreamfrequencies

Can reduce data throughput

Si l Cli i ( td)


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Signal Clipping (contd)

Noise above ~40 MHz (~65 MHz in a

Euro-DOCSIS network) is most likely caused

by laser clipping

I l N i & Cli i P k t L


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Impulse Noise & Clipping: Packet Loss

Some QAM analyzers support upstream

packet loss measurements

Graphics courtesy of Acterna, Sunrise Telecom and Tril ithic

Transient H m Mod lation


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Transient Hum Modulation

Ferrite components in network and droppassive devices

High current causes ferrite material to saturate

Switching power supply noise and

harmonics



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Hum modulation problem caused by

defective connector on customers VCR

Intermittent Connections


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Intermittent Connections

Self-induced

Network maintenance: changing pads &equalizers, amplifier modules

Craft-relatedLoose or damaged connectors

Poor quality installation

Sweep Transmitter Operation


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Sweep Transmitter Operation

Many cable operators usebroadband sweep equipmentfor network maintenance.

Sweep transmitter interference toupstream digitally modulated carriers is a

common problem. When it happens,degraded BER performance occurs.

To avoid sweep interference problems,make sure the reverse sweep transmitterhas appropriate guard bands programmedaround each upstream digitally modulatedcarrier.

Still Having Problems?


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If everything appears to check out OK inthe headend but cable modem operationalproblems still exist in the field, it may be a

cable network problem This can be verified by connecting the

CMTS to a six-foot plant

Six Foot Plant


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Six-Foot Plant

Upconverter

10 dB to

15 dB

atten.

33 dB

atten.

30 dB

atten.

Diplexfilter

8-waysplitter

Common

Low

High

Upstream

Downstream

+25 to +35 dBmVI.F. input

+55 to +58 dBmVRF output

CMTS

Cable modems

10 dB

atten.



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If CMTS configurationis correct and headend

problems have beenruled out, its time tomove to the outside

plant.

Out in the Field


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Out in the Field

Use appropriate test equipment tocharacterize the return path between thesubscriber premises and CMTS.

Out in the Field


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Out in the Field

Verify that the amplitude of the upstreamdigitally modulated carrier at the cable

modem output is in the +8 dBmV to +58dBmV range for QPSK, and +8 dBmV to+55 dBmV for 16-QAM.

Correct levels at the first upstream active?

Out in the Field


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Out in the Field

Use the divide and conquer method to

locate problems in the network

Headend Fiber Node

A CB D E

A Few Potential Problems


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Improper RF levels

Poor carrier-to-junk ratio (the DOCSISminimum spec is 25 dB for both QPSK and 16-QAM, as well as for all of the new upstreammodulation formats in DOCSIS 2.0)

Headend upstream combining/splitting

Too many nodes or homes passed per CMTS upstreamport

Upstream fiber links not correctly aligned

Forward and reverse amplifiers not correctly

aligned



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Ingress, impulse noise, spurious interference,distortions, laser clipping

Loose or intermittent connections

Hum modulation (the DOCSIS maximum spec is7%, or 23 dBc)

Microreflections (analogous to multipath orghosting in analog TV pictures)

DOCSIS 1.0 Assumed Upstream RFChannel Transmission Characteristics


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Channel Transmission Characteristics

Parameter ValueFrequency range 5 to 42 MHz edge to edgeTransit delay from the most distant CM to the

nearest CM or CMTS


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Parameter Value

Frequency 5 to 42 MHz edge to edge

Level range (one channel) +8 to +55 dBmV (16QAM)

+8 to +58 dBmV (QPSK)

Modulation type QPSK and 16QAM

Symbol rate (nominal) 160, 320, 640, 1,280 and 2,560 ksym/sec

Bandwidth 200, 400, 800, 1,600 and 3,200 kHz

Output impedance 75 ohms

Output return loss >6 dB (5-42 MHz)

Connector F connector per [IPS-SP-406] (commonwith the input)

DOCSIS 1.1 Assumed Upstream RFChannel Transmission Characteristics


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Parameter Value

Frequency range 5 to 42 MHz edge to edge

Transit delay from the most distant CM to thenearest CM or CMTS


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Parameter Value

Frequency 5 to 42 MHz edge to edge

Level range (one channel) +8 to +55 dBmV (16QAM)

+8 to +58 dBmV (QPSK)

Modulation type QPSK and 16QAM

Symbol rate (nominal) 160, 320, 640, 1,280 qne 2,560 ksym/sec

Bandwidth 200, 400, 800, 1,600 and 3,200 kHz

Output impedance 75 ohms

Output return loss >6 dB (5-42 MHz)

Connector F connector per [ISO-169-24] (commonwith the input)

Useful ReferencesMagazine Articles


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g

Hranac, R. 16-QAM Success Story. CommunicationsTechnology, September 2002

www.broadband-pbimedia.com/ct/archives/0902/0902_broadband.html

Hranac, R. More on 16-QAM. Communications Technology,January 2003

www.broadband-pbimedia.com/ct2/archives/0103/0103_broadband.html

Hranac, R. Mystified by Return Path Activation? Get YourUpstream Fiber Links Aligned. Communications Technology,March 2000

www.broadband-pbimedia.com/ct2/archives/0300/feature1.htm

Hranac R. Seek Balance in All Things: A Look at Unity Gain inthe Upstream Coax Plant. Communications Technology, June2000

www.broadband-pbimedia.com/ct2/archives/0600/0600fe8.htm



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Hranac, R. Hum Got You Down? Block Capacitors Fix ReversePath Woes Communications Technology, May 1999www.broadband-pbimedia.com/ct2/archives/0599/ct0599d.htm

Hranac, R., M. Mil let. Upstream Power Measurements: Watts UpDoc? Communications Technology, December 2000

www.broadband-pbimedia.com/ct2/archives/1200/064_upstream.htm

Hranac, R. Two-Way Success Secrets Revealed CommunicationsTechnology, April 2001

www.broadband-pbimedia.com/ct2/archives/0401/034_broadband.htm



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Hranac, R. Understanding Reverse Path Problems, Part 1Communications Technology, September 2000www.broadband-pbimedia.com/ct2/archives/0900/0900col01.htm

Hranac, R. Understanding Reverse Path Problems, Part 2Communications Technology, October 2000www.broadband-pbimedia.com/ct2/archives/1000/032_broadband.htm

Hranac, R. Understanding Reverse Path Problems, Part 3Communications Technology, November 2000www.broadband-pbimedia.com/ct2/archives/1100/038_broadband.htm



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From back issues prior to January 1999 (not available in publishers

on-line archives):

Hranac, R. A Unique Approach to Reverse Path TestingCommunications Technology, December 1997

Hranac, R. Impulse Noise in Two-Way Systems CommunicationsTechnology, July 1996

Hranac, R. Combating Impulse Noise With Common Mode

Suppression Communications Technology, August 1996

Hranac, R. Passive Device Intermod CommunicationsTechnology, September 1998

Useful ReferencesBooks


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Farmer, J., D. Large, and W. Ciciora. Modern CableTelevision Technology: Video, Voice and DataCommunications. Morgan Kaufmann Publishers; 1998

Raskin, D. and D. Stoneback. Broadband ReturnSystems for Hybrid Fiber/Coax Cable TV systems.Prentice Hall; 1997

Thomas J.L. Cable Television Proof of Performance: APractical Guide to Cable TV Compliance MeasurementsUsing a Spectrum Analyzer. Prentice Hall; 1995.

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