Downsream RF Troubleshooting

8/12/2019 Downsream RF Troubleshooting

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111 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID


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2 2001, Cisco Systems, Inc. All rights reserved.

Session Number

Presentation_ID

Downstream RFTroubleshooting

Ron Hranac

Technical Leader, Broadband Network Engineering


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CMTS Configuration

Check this firstincorrect CMTSconfiguration is a very common problem!


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5/32555 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID

Upconverter Setup

CMTS

RF upconverter

88-860 MHz downstream

RF output to CATV network

(+50 dBmV to +61 dBmV)

Attenuator44 MHz downstream

IF output

(e.g., +42 dBmV +/-2 dB)

44 MHz IF input to

upconverter(typ. +25 dBmV to +35

dBmV)



CMTS Downstream IF Output

Measure the CMTSs downstreamintermediate frequency (IF) output level

Verify that the level meets the CMTSmanufacturers spec (e.g., +42 dBmV, +/-2

dB)



Upconverter IF Input

Measure the IF input level to the externalupconverter

Refer to the upconverter manufacturers

specifications for the correct IF input levelTypically in the range of +25 dBmV to +35dBmV

In most cases an in-line attenuator ( pad ) wil lbe required between the CMTS IF output andupconverter IF input



Upconverter Operating Parameters

Refer to the upconverter manufacturersspecifications for correct setup guidelines

IF and RF operating levels

IF AGC

Digital modulation versus analog modulationOutput frequency



Upconverter Output Frequency

Digitally modulated carrier frequency isusually defined by the channels center

frequency For example, EIA channel 71 (504-510

MHz) has a center frequency of 507.0 MHz

Most upconverters display the desiredchannels center frequency

An exception is the GI/Motorola C6Uitsfrequency readout displays the equivalentanalog TV channel visual carrier frequency.

For example, Ch. 71 would be displayed as505.25 MHz rather than 507.0 MHz.



Upconverter RF Output

Measure the upconverters radiofrequency (RF) output level

DOCSIS specifies +50 dBmV to +61 dBmVcapability

Typical operation is in the +55 dBmV to

+58 dBmV range, just like a processor ormodulator



Digitally Modulated Carrier Amplitude

Verify that the digitally modulated carriers

average power level is 6 dB to 10 dB belowthe amplitude of what an analog TVchannel on the same frequency would be


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QAM Analyzer

A QAM analyzer is agood troubleshootingtool

Most provide pre- andpost-FEC BER

measurements

MER also is available


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QAM Analyzer

The constellation displayis perhaps a QAM

analyzers most usefulfunction

This is an example of a

good 256-QAMconstellation. The dotclusters are tight andwell-defined, and are

located away fromdecision boundaries. Theoverall shape of the 256

dot clusters forms asquare.


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QAM Analyzer

Use the QAM analyzer to check the CMTS

IF output, upconverter IF input, andupconverter RF output: MER, pre- andpost-FEC BER and constellation

CMTS

RF upconverterAttenuator


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Modulation Error Ratio

Modulation error ratio (MER) is analogousto baseband signal-to-noise ratio

The recommended minimum MER for64-QAM is 27 dB

The recommended minimum MER for

256-QAM is 31 dB


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Modulation Error Ratio

MER in the headendCMTS output,upconverter input and output, and

downstream laser inputshould be in the34 dB to 36 dB or greater range.


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Bit Error Rate

Ideally there should be no bit errors in thedownstream digitally modulated carrier

DOCSIS specifies the cable modem post-FEC BER to be 1x10-8 or less


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Sweep Transmitter Operation

Many cable operators usebroadband sweep equipment

for network maintenance. Sweep transmitter interference to

downstream digitally modulated carriers is

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

To avoid sweep interference problems,make sure the sweep transmitter hasappropriate guard bands programmed

around each downstream digitallymodulated carrier.


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One More Thing to Check

Measure RF levels, MER and BER at thedownstream laser input, to make sure

other headend equipment such ascombining amplifiers havent impaired thedigitally modulated carrier

Verify that downstream laser transmitterRF input levels are within spec

Excessive levels can cause downstream laserclipping, which may not be visible in theanalog TV pictures. However, clipping willdegrade the downstream digitally modulatedcarriers BER performance.


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Still Having Problems?

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


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Six-Foot Plant

Upconverter

10 dB to

15 dB

atten.

30 dB

atten.

30 dB

atten.

Diplexfilter

8-waysplitter

Common

Low

High

Upstream

Downstream

+25 to +35 dBmV

I.F. input

+55 to +58 dBmV

RF output

CMTS

Cable modems

10 dB

atten.


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Still Having Problems?

If CMTS configurationis correct and headend

problems have beenruled out, its time tomove to the outside

plant.


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Out in the Field

Go to the fiber node thatserves the affected areaand measure RF levels,MER and BER, and lookfor constellation

impairments. Problemsherebut not in theheadendsuggest

something may be amissin the downstream fiberlink.


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Out in the Field

If everything checks out OK at the node,

go to an affected subscribers premises. Measure RF levels, MER and BER, and

evaluate the constellation for impairments.


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Out in the Field

Verify that the amplitude (average powerlevel!) of the downstream digitallymodulated carrier at the cable modeminput is in the 15 dBmV to +15 dBmVrange.

The total RF powerthat is, the

contribution of the entire downstream RFspectrumat the cable modem input mustbe less than +30 dBmV .


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A Few Potential Problems

Improper RF levels

Poor carrier-to-noise ratio (the DOCSIS minimumspec is 35 dB for both 64- and 256-QAM)

Loose or intermittent connections

Hum modulation (the DOCSIS maximum spec is5%, or 26 dBc)

Ingress, impulse noise, spurious interference,distortions

Microreflections (analogous to multipath or

ghosting in analog TV pictures)


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Something Else to Check

Poor in-channelfrequency response maycause excessive group

delay, which will degradeBER performance.

Set the spectrum

analyzer to 3 dB/divisionand place the top of thecarrier in an upper

graticule. Set RBW to 30kHz, VBW to 1 kHz, andturn on video averaging.The analyzer will display

approximate in-channelfrequency response.

The DOCSIS 1.1 spec for in-channel

frequency response (amplitude ripple)

is 3 dB. This is an example of goodfrequency response.

DOCSIS 1 0 Downstream RF Channel


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DOCSIS 1.0 Downstream RF ChannelTransmission Characteristics

Parameter ValueFrequency range Cable system normal downstream operating

range is from 50 MHz to as high as 860 MHz.However, the values in this table apply only atfrequencies >= 88 MHz.

RF channel spacing (design bandwidth) 6 MHz

Transit delay from headend to most distantcustomer


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DOCSIS 1.0 Electrical Input to CableModem

Parameter ValueCenter Frequency 91 to 857 MHz, 30 kHz

Level Range (one channel) -15 dBmV to +15 dBmV

Modulation Type 64-QAM and 256-QAM

Symbol Rate (nominal) 5.056941 Msym/sec (64-QAM) and5.360537 Msym/sec (256-QAM)

Bandwidth 6 MHz (18% Square Root Raised Cosineshaping for 64-QAM and 12% Square RootRaised Cosine shaping for 256-QAM)

Total Input Power (40-900 MHz) < 30 dBmV

Input (load) Impedance 75 ohms

Input Return Loss > 6 dB (88-860 MHz)

Connector F connector per [IPS-SP-406] (common withthe output)

DOCSIS 1.1 Downstream RF Channel


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DOCSIS 1.1 Downstream RF ChannelTransmission Characteristics

Parameter ValueFrequency range Cable system normal downstream operating range is from 50 MHz to as high

as 860 MHz. However, the values in this table apply only at frequencies >=88 MHz.

RF channel spacing (design bandwidth) 6 MHzTransit delay from headend to most distantcustomer


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DOCSIS 1.1 Electrical Input to CableModem

Parameter Value

Center Frequency 91 to 857 MHz, 30 kHzLevel Range (one channel) -15 dBmV to +15 dBmV

Modulation Type 64-QAM and 256-QAM

Symbol Rate (nominal) 5.056941 Msym/sec (64-QAM) and5.360537 Msym/sec (256-QAM)

Bandwidth 6 MHz (18% Square Root Raised Cosine

shaping for 64-QAM and 12% Square RootRaised Cosine shaping for 256-QAM)

Total Input Power (40-900 MHz) < 30 dBmV

Input (load) Impedance 75 ohms

Input Return Loss > 6 dB (88-860 MHz)

Connector F connector per [ISO-169-24] (common withthe output)

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Downsream RF Troubleshooting