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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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333 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
CMTS Configuration
Check this firstincorrect CMTSconfiguration is a very common problem!
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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)
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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)
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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
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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
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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.
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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
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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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121212 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
QAM Analyzer
A QAM analyzer is agood troubleshootingtool
Most provide pre- andpost-FEC BER
measurements
MER also is available
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131313 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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141414 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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171717 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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181818 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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191919 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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202020 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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212121 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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222222 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
Still Having Problems?
If CMTS configurationis correct and headend
problems have beenruled out, its time tomove to the outside
plant.
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232323 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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313131 2001, Cisco Systems, Inc. All rights reserved.Presentation_ID
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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Presentation_ID 323232 2001, Cisco Systems, Inc. All rights reserved.
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