Return Path Issues and Answers Rev. # 3 Feb. 2002

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

HCX comController

Router

Analog Video55-319MHz

2 WayRF splitter

430MHz

600MHz

Headend Combining

Analyzer

RX RX RX

Headend Combining

Analyzer

RX RX RX

Energy AccumulationEnergy Accumulation

Return Path SignalReturn Path Signal

Funnel Effect Cont..

Combiner

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

CMTS Demod

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

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

Combining Network

FCNCLEAR

status

abc def ghi

jkl mno pqr

stu vwx yz

space +/-

TILT SCANLEVEL

C/N HUM MOD

System Sweep Transmitter 3SRSystem Sweep Transmitter 3SR Stealth SweepStealth Sweep

System Sweep Receiver Model 3SR

LEVEL TILT SCAN SWEEP

C/N HUM MOD SPECT

FCNCLEAR

status

abc def ghi

jkl mno pqr

stu vwx yz

space +/-

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

Diplex

NRICNRIK

+8 dBNRT

(Input to TP @min atten)

+39 dBmV

• 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

Optical Transmitter

FCNCLEAR

status

abc def ghi

jkl mno pqr

stu vwx yz

space +/-

TILT SCANLEVEL

C/N HUM MOD

System Sweep Transmitter 3SRSystem Sweep Transmitter 3SR Stealth SweepStealth Sweep

C/N HUM MOD SPECT

FCNCLEAR

status

abc def ghi

jkl mno pqr

stu vwx yz

space +/-

Optical Receiver

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

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)

Headend

“X”dB“X”dB

Level applied to return amp input

(Take into accountThe test point loss and

the Amplifier embedding

“X”

C/N HUM MOD SPECT

FCNCLEAR

status

abc def ghi

jkl mno pqr

stu vwx yz

space +/-

15dBmV

Set Equaliserto get equal signal

levels at both frequencies in

Head End

“X”dB

HeadendSystem Sweep Receiver Model 3SR

C/N HUM MOD SPECT

FCNCLEAR

status

abc def ghi

jkl mno pqr

stu vwx yz

space +/-

15dBmV

Set Attenuatorto get correctsignal level in

Head End

Headend System Sweep Receiver Model 3SR

C/N HUM MOD SPECT

FCNCLEAR

status

abc def ghi

jkl mno pqr

stu vwx yz

space +/-

In-Home Signal Losses

-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

5-LER-91

Network Amplifier

+15 dBmV

+22 dBmV

Plant Actives - Type AmpsRelative to Return Amp. Input

+42dBmV

Plant Actives - LERelative to Return Amp. Input

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 +15 dBmV+15 dBmV

+35 dBmV

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

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

-20dB TP @ +42 dBmV

+22 dBmV

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

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

Check at various points in the system to locate source

of ingress or noise

tap tap

To Headend

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

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 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.

Return Path Issues and Answers Rev. # 3 Feb. 2002

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