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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

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slide2

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.
slide3

Return Path Alignment Steps

  • 1. 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

26

Analog Video

Com21

55-319MHz

23

splitter

2 Way

RF

20

430MHz

2 Way

HCX comController

RF splitter

600MHz

17

Public switch

14

Return System
headend combining

RX

RX

RX

Sweep

Modem

RX

RX

RX

Phone

Analyzer

RX

RX

RX

Headend Combining
headend combining1

RX

RX

RX

Sweep

Modem

RX

RX

RX

Phone

Analyzer

RX

RX

RX

Headend Combining
energy accumulation
Energy Accumulation

Return Path Signal

funnel effect cont1

CPU

Monitoring

Device

Combiner

Funnel Effect Cont..
system behavior
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 behavior1

Return Rx

Return TX

System behavior

Average case

system behavior2
System behavior

Noise funneling (amplifiers + optics)

Return Rx

Return TX

system behavior3

Return Rx

Return TX

System behavior

noise from modem

system behavior4
System behavior

Laser Clip

Return Rx

Return TX

ingress example
Ingress Example
  • 70 % from the home
  • 25% from the drop cable

Ingress/ noise

content

Noise / ingress content

return path alignment

Return path alignment

Technical Support

slide21

Return System Components

Head-End RX

TX

Head-End RX

Head-End RX

Head-End RX

Combiner

CMTS Demod

2

3

1

5

4

7

6

optical return path link

Node

Rx

ForwardTX

Headend

Return TX

Return Rx

Optical return path link
  • Optimum level
    • input level too low ==> low thermal and RIN CNR
    • input level too high ==> high Intermodulation noise ==> low CNR

CNR

BER

input level

input level

optimum level

optimum level

alignment in the field 2
Alignment in the Field (2)

Or Spectrum Analyzer with

Video Out Function

Or Baseband Output of

Analyzer into Modulator

alignment in the field 3

Combining

Network

Node

TP

TP

Out

System Sweep Transmitter 3SR

System Sweep Transmitter 3SR

Stealth Sweep

Stealth Sweep

help

FILE

FREQ

abc

def

ghi

1

2

3

status

AUTO

jkl

pqr

4

5

mno

6

CHAN

alpha

yz

stu

8

vwx

9

ENTER

7

SETUP

x

light

.

+/-

space

CLEAR

FCN

0

PRINT

SWEEP

LEVEL

SCAN

TILT

SPECT

MOD

C/N

HUM

10 40

Alignment In the Field #3
node hub return link1
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
slide28

Optimum drive levels for the NRT

+8 dBmV/ch

Ch. width =1.6 MHz

(42-5)/1.6=23 channels

10*log(23ch)=13.6 dB

+2dBmv total = -12 dBmV/ch

+24dBmv total = +10dBmV/ch

slide29

Based on Channel Bandwidth

(42-5)/1.6=23 channels

10*log(23ch)=13.6 dB

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 [email protected] 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)

node adjustment

Specified level into forward TP is 39dBmV

Node adjustment

Test point sticker level

is level for video carriers

=> for digital, target is

TP level is 8dB

Corresponding input level

is 19dBmV

(20dB)

slide31

NRT Field Alignment

(From the GNA Installation manual)

  • 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

Combining

Network

Node

Optical Receiver

System Sweep Transmitter 3SR

System Sweep Transmitter 3SR

Stealth Sweep

Stealth Sweep

help

FILE

FREQ

abc

def

ghi

1

2

3

status

AUTO

jkl

pqr

4

5

mno

6

CHAN

alpha

yz

stu

8

vwx

9

ENTER

7

SETUP

x

light

.

+/-

space

CLEAR

FCN

0

PRINT

SWEEP

LEVEL

SCAN

TILT

SPECT

MOD

C/N

HUM

Stealth Reverse Sweep

Optical Transmitter

TP

Out

Optical Receiver

3ST

Reverse Sweep Displayed on 3SRV

3SRV

slide35

Return Path Requirements

Signal Levels

Passive Values

Unity Inputs

slide36

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
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)

Determine Return Input Levels
  • What return amplifier inputs

are required?

procedure
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

Ref

Head End Reference

18dBmV

49dBmV

Note Reference levels at Headend and retain for rest

of amp chain

(Start with longest link)

return amplifier set up

15dBmV

L

“X”

Return Amplifier Set-Up

Headend

Ref

“X”dB

“X”dB

Level applied to

return amp input

(Take into account

The test point loss and the

Amplifier embedding loss)

Output

Equaliser

(per map

Design)

Output

Attenuator

(per map

design)

return amplifier set up1
Return Amplifier Set-Up

Headend

15dBmV

Ref

“X”dB

Set Equaliser

to get equal signal

levels at both

frequencies in

Head End

return amplifier set up2
Return Amplifier Set-Up

Headend

15dBmV

Ref

Set Attenuator

to get correct

signal level in

Head End

slide45

Example of losses at 40MHz

Drop -2.1 dB

Splitter -3.5 dB

RG59 -0.8 dB

Splitter -3.5 dB

RG59 -0.8 dB

=============

Total -10.7 dB

Tap

150\'

RG-6

-2.1 dB

-3.5 dB

50\'

-0.8 dB

50\'

-0.8 dB

-3.5 dB

In-Home Signal Losses

We will use -10dB as the typical in-house and drop loss.

slide46

RF Plant Passive Losses

Relative to Return Amp Input

7 dBmV

embedding loss

+15 dBmV

at amp input

Cable Losses

+23 dBmV

+25 dBmV

@870 MHz

+28 dBmV

@40 MHz

-5

-1.0

-7

-11

-2.0

-3.0

26

23

20

17

+22 dBmV

needed at

Input to Housing

+45 dBmV

into tap port

+15 dBmV

at amp input

+48 dBmV

into tap port

A= Closest to node

High tap Value

B= Farthest from node

low tap value

-10 dB

internal and

drop loss

-10 dB

internal and

drop loss

+55 dBmV

Modem output

+58 dBmV

Modem output

slide47

+22 dBmV

+15 dBmV

Plant Actives - Type AmpsRelative to Return Amp. Input

+42dBmV

H

H

0

L

L

H

0

L

5-LER-91

H

0

L

-2 dB

-7 dB

Network Amplifier

slide48

H

H

0

L

L

+17 dBmV

5-LER-91

+15 dBmV

-2 dB

-2 dB

Line Extender

Plant Actives - LERelative to Return Amp. Input

+47dBmV

+37dBmV

return set up relative to return amplifier input

-30dB TP @ +52 dBmV

+35 dBmV

+35 dBmV

H

H

H

0

0

L

L

L

H

H

0

0

L

L

H

H

+15 dBmV

0

0

L

Input to Type Return Amp. = 15dBmV

Amp. Embedding Losses = 7dB

Cable Loss at 40MHz = 6dB

Diplex Filter Loss = 2dB

Station Gain = 24dBmV

Input Level to Return Amp. = 15dBmV

+47 dBmV

+ 35 dBmV

23

H

H

0

L

L

+17 dBmV

40 dBmV

+15 dBmV

Return Set Up relative to Return Amplifier Input

-20dB TP @ +42 dBmV

H

+22 dBmV

L

+22 dBmV

9 Pad

5 EQ.

+15 dBmV

L

2 Pad

5 EQ.

To TX Input at Node

15 dBmV Input Level

Input to TX =15dBmV

Node Embedding Losses = 14dB

Cable Loss at 40MHz = 6dB

Diplex Filter Loss = 2dB

Station Gain = 24dBmV

Input Level to Return Amp. = 15dBmV

9 Pad

5 EQ.

common mode distortion
Common Mode Distortion
  • 6 MHz Beats
  • Cause
  • Location
common mode distortion1
Common Mode Distortion

REF 6.0 dBmV

MKR 8.90 MHz

-15.93 dBmV

ATTEN 10 dB

PEAK

LOG

5 dB/

0 Hz

RES BW 300 KHz

40.00 MH

SWP 20 msec

VBW 100 KHz

common path distortion1

Modem Return

Spectrum

25dB C/N

@ Status Monitoring

Frequency

Common Path Distortion
common problems1

Terminator

AC Blocking Terminator

Common Problems
introduction to return path testing
Introduction to Return Path Testing
  • Testing on the return path is significantly different than the forward path.
  • Ingress from anywhere in the node can effect all subscribers on that node and interfere with data traffic.
  • Subscriber’s modems must time share bandwidth on the return with all other users on that node.
  • 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

To Headend

tap

tap

tap

tap

tap

tap

Node

Using a Spectrum Display to Track Ingress and Noise

Return Modem Signal

  • Use a spectrum analyzer display to track the source of noise and ingress in the system.

Return Modem Signal

Check at various points in the system to locate source of ingress or noise

Noise or Ingress

limitations of spectrum displays for catching fast transients
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 transients1
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 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 function1
Max Hold Function

Max Hold Trace

Current Sweep

forward return interaction
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
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?
docsis information
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 path issues and answers1
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
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