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Return Path Issues and Answers
Rev. # 3 Feb. 2002
Return System Design and Operational Goals
Operate the Return TX at its “optimum” drive level.
• Optimum is based on the maximum TOTAL power at the TX
Align the return amplifiers so they all provide the same signal levels at the node input.
• Set the amplifiers for “Unity Gain”
Adjust the modems so they all provide the same signal levels at the amplifier inputs.
• Modem transmit levels are controlled by long loop AGC based on the receive level at the head-end
• The modem with the longest (dB) return path must be capable of reaching the head-end demodulator.
Return Path Alignment Steps1. Determine the optimum drive level at the laser,
2. Inject an equivalent level reference signal at the transmitter.
3. Adjust receiver output level and head-end combining to achieve proper levels at the CMTS demodulator.
4. Establish reference levels at the CMTS demodulator, or other head-end reference point.
5. Determine the optimum RF input level for the RF actives.
6. Adjust return amps for unity gain. Work from node outward, inject known levels at the RF amp input, adjust gain and equalizer to get the same reference levels at the head-end.
When the modem demodulator has the proper level, the optical transmitter will be operating at optimum drive level.
Return System
Public switch
Com21
HCX comController
Router
CM TS
Analog Video55-319MHz
2 W
ayR
Fs
plit
ter
2 WayRF splitter
430MHz
600MHz
26
23
20
17
14
Headend Combining
Sweep
Modem
Phone
Analyzer
RX RX RX
RX RX RX
RX RX RX
Headend Combining
Sweep
Modem
Phone
Analyzer
RX RX RX
RX RX RX
RX RX RX
Energy AccumulationEnergy Accumulation
Return Path SignalReturn Path Signal
Funnel Effect Cont..
Funnel Effect Cont..
Combiner
CPU
MonitoringDevice
System behavior
• Thermal noise funneling– Most important cause of thermal noise:
• 1) subscribers
• 2) amplifiers
• 3) optical link
• Laser Clip
• Ingress and Impulse Noise
System behavior
Return TXReturn Rx
Average case
Return Noise Floor
System behavior
Return TXReturn Rx
Noise funneling (amplifiers + optics)
Return Noise Floor
System behavior
noise from modem
Return TXReturn Rx
System behavior
Return TXReturn Rx
Laser Clip
Ingress ExampleIngress Example• 70 % from the home• 25% from the drop cable
Ingress/ noise content
Noise / ingress content
Random Noise VS. Time
Impulse Noise
Return path alignment
Technical Support
Return System ComponentsTX
Head-End RX
Combiner
Head-End RX
Head-End RX
Head-End RX
CMTS Demod
7
6
5
4
3
2
1
Optical return path link
• Optimum level– input level too low ==> low thermal and RIN CNR– input level too high ==> high Intermodulation noise
==> low CNR
RxForward TX
Return TXReturn RxHeadend
Node
input level
CNR BER
optimum level optimum levelinput level
Node-Hub Return Link
Alignment in the Field (1)
Person 1Person 1
Alignment in the Field (2)
Or Baseband Output ofOr Baseband Output of
Analyzer into ModulatorAnalyzer into Modulator
Or Spectrum Analyzer with Or Spectrum Analyzer with
Video Out FunctionVideo Out Function
10 40
Combining Network
Out
FREQ
CHAN
ENTER
FCNCLEAR
help
status
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1 2 3
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FILE
AUTO
SETUP
TILT SCANLEVEL
C/N HUM MOD
SWEEP
SPECT
System Sweep Transmitter 3SRSystem Sweep Transmitter 3SR Stealth SweepStealth Sweep
Node
TPTP
System Sweep Receiver Model 3SR
LEVEL TILT SCAN SWEEP
C/N HUM MOD SPECT
FILE
AUTO
SETUP
FREQ
CHAN
ENTER
FCNCLEAR
help
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Alignment In the Field #3Alignment In the Field #3
Node-Hub Return Link
• Set up link to carry max (example) 23 (QPSK) ch–OT drive spec for 2 Video channels 10 - 20 dBmV–optimum for 4 ch = 10Log(2/4) = -3 dB reduction in
drive level
• Apply 2 carriers at “X”dBmV to node• Adjust gain of node return transmitter to obtain
correct drive level• Measure received Hub optical power• Measure RF out from Hub receiver
Optimum drive levels for the NRT
+24dBmv total = +10dBmV/ch+2dBmv total = -12 dBmV/ch
10*log(23ch)=13.6 dB
Ch. width =1.6 MHz
(42-5)/1.6=23 channels
+8 dBmV/ch
Drive Levels for the NRT Current factory alignment procedure
• Aligned with two CW carriers
•Reference drive level is listed on the sticker - as measured at the transmitter testpoint. Typically +18 dBmV
• Total voltage at clip point approximately 2ch.@ 18dBmV = 24dBmV
( 20*log(2)=6 )
• QPSK Channel width =1.6 MHz
22-13.6= 8.4dB per Channel
(22dBmV value 2dBmV below QAM Clip)
(42-5)/1.6=23 channels
10*log(23ch)=13.6 dB
Based on Channel BandwidthBased on Channel Bandwidth
Node adjustmentTest point sticker levelis level for video carriers=> for digital, target is TP level is 8dB
Specified level into forward TP is 39dBmV
Corresponding input levelis 19dBmV
(20dB)
NRT Field Alignment(From the GNA Installation manual)
-6 dB-5 dB -3 dB
H
L
Diplex
0
NRICNRIK
+8 dBNRT
-5
(Input to TP @min atten)
+39 dBmV
-20dB
+19
+5+8
• Field alignment is done at “digital” levels, but using CW carriers.
• NTR gain is set “mid-range”, or -5dB.
• To get +8dBmV at the TP, +19dBmV is required at the node input ports.
• With a 20dB testpoint, a signal level of +39dBmv is injected at the node input TP.
Stealth Reverse Sweep
Reverse Sweep Displayed on 3SRV
Combining Network
Out
Optical Transmitter
FREQ
CHAN
ENTER
FCNCLEAR
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status
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FILE
AUTO
SETUP
TILT SCANLEVEL
C/N HUM MOD
SWEEP
SPECT
System Sweep Transmitter 3SRSystem Sweep Transmitter 3SR Stealth SweepStealth Sweep
Node
System Sweep Receiver Model 3SR
LEVEL TILT SCAN SWEEP
C/N HUM MOD SPECT
FILE
AUTO
SETUP
FREQ
CHAN
ENTER
FCNCLEAR
help
status
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3SRV
Optical Receiver
Optical Receiver
TP
3ST
Reverse Sweep Reference
Reverse Sweep
Return Path RequirementsReturn Path RequirementsReturn Path RequirementsReturn Path Requirements
Signal LevelsPassive Values
Unity Inputs
Signal Level Requirements at the RF actives
The next step is to adjust all the RF actives for unity gain, but first you need to determine the desired RF input levels.
• In general, you want the return signal to be high relative to system ingress.
• What signal level can be expected at the RF amplifier when the modem with the highest loss path transmits at its highest power?
Signal Level Requirements at the RF actives
• System should be designed for constant input level whether at the STATION ports or at the Input to the Return Amp..
• Amplifiers are aligned for unity gain back to the Node, by inserting a reference signal and adjusting for the proper received level at the head-end.
• Internal combining losses should be taken into account when determining the correct CW carrier level to use as the reference signal.
Determine Return Input Levels
• Carrier to Noise at Transmitter
• Noise Figure Return Amp.
• Total Node Actives
• C/N Total = C/N single-10Log N
• C/N single = Input + 59 – N.F.
• -50 dbc• 5 dB• 75 Actives• --50 = X – 10(LOG 75)• -50 = X – 18.75• X = -50 + -18.75 = -
68.75dbc• -68.75 = X + 59 – 5• -68.75 = X + 54• X = 54 – (-68.75)• X = 14.75 dB(15)
What return amplifier inputsWhat return amplifier inputs
are required?are required?
Return path alignment example
Procedure
• Set-up RF Amps
– Start with amplifier closest to node and work out
– Return amplifier has specified input level for a given channel plan
– Apply return input and adjust to obtain reference levels at headend
Head End Reference
49dBmV18dBmV
Ref
Note Reference levels at Headend and retain for restof amp chain
(Start with longest link)
Return Amplifier Set-Up
15dBmV
Output Equaliser(per mapDesign)
Output Attenuator(per map design)
Ref
Headend
“X”dB“X”dB
Level applied to return amp input
(Take into accountThe test point loss and
the Amplifier embedding
loss)
L
“X”
System Sweep Receiver Model 3SR
LEVEL TILT SCAN SWEEP
C/N HUM MOD SPECT
FILE
AUTO
SETUP
FREQ
CHAN
ENTER
FCNCLEAR
help
status
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Return Amplifier Set-Up
15dBmV
Set Equaliserto get equal signal
levels at both frequencies in
Head End
“X”dB
Ref
HeadendSystem Sweep Receiver Model 3SR
LEVEL TILT SCAN SWEEP
C/N HUM MOD SPECT
FILE
AUTO
SETUP
FREQ
CHAN
ENTER
FCNCLEAR
help
status
alpha
light
abc def ghi
jkl mno pqr
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4 5 6
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Return Amplifier Set-Up
15dBmV
Set Attenuatorto get correctsignal level in
Head End
Ref
Headend System Sweep Receiver Model 3SR
LEVEL TILT SCAN SWEEP
C/N HUM MOD SPECT
FILE
AUTO
SETUP
FREQ
CHAN
ENTER
FCNCLEAR
help
status
alpha
light
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In-Home Signal Losses
Tap
150'
-2.1 dBRG-6
50'-0.8 dB
-3.5 dB 50'-0.8 dB
-3.5 dB
Drop -2.1 dB
Splitter -3.5 dB
RG59 -0.8 dB
Splitter -3.5 dB
RG59 -0.8 dB
=============
Total -10.7 dB
Example of losses at 40MHz
We will use -10dB as the typical in-house and drop loss.
RF Plant Passive LossesRelative to Return Amp Input
-7-2.0
-11-3.0
-5-1.0
+55 dBmVModem output
-10 dBinternal and drop loss
+45 dBmVinto tap port
+28 dBmV+25 dBmV+23 dBmV
+58 dBmVModem output
-10 dBinternal and drop loss
+48 dBmVinto tap port
23 20 17
Cable Losses
@870 MHz
@40 MHz26
+15 dBmVat amp input
A= Closest to nodeHigh tap Value
+15 dBmVat amp input
B= Farthest from nodelow tap value
7 dBmVembedding loss
+22 dBmVneeded atInput to Housing
H
L0
H
L0
H
L0
-7 dB
H
L
-2 dB
5-LER-91
Network Amplifier
+15 dBmV
+22 dBmV
Plant Actives - Type AmpsRelative to Return Amp. Input
Plant Actives - Type AmpsRelative to Return Amp. Input
+42dBmV
Plant Actives - LERelative to Return Amp. Input
Plant Actives - LERelative to Return Amp. Input
HL
0
-2 dB
HL
-2 dB
5-LER-91
Line Extender
+17 dBmV
+15 dBmV
+47dBmV
+37dBmV
Return Set Up relative to Return Amplifier Input
+22 dBmV
+35 dBmV
-30dB TP @ +52 dBmV
HL0
HL0
HL0 +15 dBmV+15 dBmV
+35 dBmV
HL0
HL0
H0
HL
To TX Input at Node15 dBmV Input Level
Input to TX =15dBmVNode Embedding Losses = 14dBCable Loss at 40MHz = 6dBDiplex Filter Loss = 2dBStation Gain = 24dBmVInput Level to Return Amp. = 15dBmV
2 Pad5 EQ.
+ 35 dBmV
HH0 LL
+17 dBmV
+15 dBmV
+47 dBmV
23
40 dBmV
Input to Type Return Amp. = 15dBmVAmp. Embedding Losses = 7dBCable Loss at 40MHz = 6dBDiplex Filter Loss = 2dBStation Gain = 24dBmVInput Level to Return Amp. = 15dBmV
HL
-20dB TP @ +42 dBmV
L
+22 dBmV
9 Pad5 EQ.
9 Pad5 EQ.
Common Mode Distortion
• 6 MHz Beats
• Cause
• Location
Common Mode DistortionCommon Mode Distortion
VBW 100 KHzVBW 100 KHz40.00 MH40.00 MH
SWP 20 msecSWP 20 msec0 Hz0 HzRES BW 300 KHzRES BW 300 KHz
PEAKPEAKLOGLOG5 dB/5 dB/
ATTEN 10 dBATTEN 10 dBMKR 8.90 MHzMKR 8.90 MHz
-15.93 dBmV-15.93 dBmVREF 6.0 dBmVREF 6.0 dBmV
Common Path Distortion
6MHz FSK Interfering Beat
Common Path Distortion
25dB C/N@ Status Monitoring Frequency
Modem Return Spectrum
Coax
Seizure
Common ProblemsCommon Problems
Connector
Common Problem Results
Common Problems
Terminator
AC Blocking Terminator
Return Noise Floor
Return Path Measurements
Introduction to Return Path Testing
1. Testing on the return path is significantly different than the forward path.
2. Ingress from anywhere in the node can effect all subscribers on that node and interfere with data traffic.
3. Subscriber’s modems must time share bandwidth on the return with all other users on that node.
4. Spectrum displays of a spectrum analyzer are very useful tools for analyzing the return path and the signals carried on it.
Using a Spectrum Display to Track Ingress and Noise
• Use a spectrum analyzer display to track the source of noise and ingress in the system.
Noise or Ingress
Return Modem Signal
Return Modem Signal
Check at various points in the system to locate source
of ingress or noise
tap tap
Node
To Headend
tap tap
tap tap
Limitations of Spectrum Displays for Catching Fast Transients.
• Scanning Spectrum Analyzers measure only one band of frequencies at any given instant.
Frequency Range Where Measurement is Being Made at That Instant
Frequencies Stored From Last Pass of
Filter
Limitations of Spectrum Displays for Catching Fast Transients.
• If the spectrum analyzer is at another frequency when the transient appears it will not be displayed.
A transient happening at this time will be missed by the filter unless it is still there when the filter comes by again
Max Hold Function
• Max Hold allows the spectrum display to catch transient signals such as ingress and modems.
• Max hold displays the highest level measured and holds it until the trace is cleared by the user or a setting changed.
• Max hold will only catch a transient if it is present at the time the sweep passes the frequency of the transient.
• Allowing the trace to build up over time using max hold increases the chance of catching fast transients.
Max Hold Function
Current Sweep
Max Hold Trace
Forward & Return Interaction
• An increase in forward levels can create distortions that fall within the return path bandwidth– This will appear on an
analyzer to be poor diplex filter isolation or common path distortion (CPD)
Setting Up For Certification
• A few words about test equipment– If it’s not calibrated - it’s not valid– Know your gear - what does it need to give you
accurate results?
Performing Certification Tests
Performing Certification Tests
Performing Certification Tests
Performing Certification Tests
Performing Certification Tests
Performing Certification Tests
DOCSIS Information• To move from QPSK to 16 QAM requires an
increase in CNR of ~7 dB to maintain a given BER
• To move from 16 QAM to 64 QAM an additional increase in CNR of ~6 dB is required to maintain the same BER
• The more complex the modulation format, the more error prone it becomes to transmission impairments
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Shots From The Field
Return Path Issues and Answers• Follow the Manufacturers
Guidelines and Specifications.• Complete Headend Combining
Prior to Activation.• Start with the Furthest Node
and work toward the Headend.• Align all Nodes Identically.• Adjust Optic Receivers to
accommodate the Termination Equipment.
• Check Return for Noise and Distortions.
• Set the return actives for Unity Gain.
• Know the in home devices capability and operational range.
• Maintain the System Integrity.