Larry Jump Field Applications Engineer Sunrise Telecom Broadband January 22, 2004

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Larry JumpField Applications EngineerSunrise
Telecom BroadbandJanuary 22, 2004

• Return Path Basics

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My Business Card

• Larry Jump
• Sales Support Engineer
• Sunrise Telecom Broadband
• 156 James Hill Rd
• Port Matilda, PA 16870
• 814-692-4294
• ljump_at_sunrisetelecom.com
• www.sunrisetelecom.com

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Purpose

• An understanding of the basics of return path
  operation
• To ensure that you know how to align it properly
• What are the signal impairments on the reverse
  path
• Hints on Return Path Troubleshooting

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Agenda

• A little bit of history
• Unity Gain concept
• Design Concepts
• Different alignment techniques of the past
• Return Sweeping
• Impairments and Troubleshooting

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In Ancient Times we did it this way.

• Minimal use in the past (local events and
  fulfilling franchise agreements)
• Technical problem with the funneling issue with
  the tree and branch architectures.
• Fiber HFC networks reduced the technical issues
  and made use of the return path possible.
• Drive to gain revenue from existing network
  investment by adding new services which require
  two-way communications.
• Industry wide, we knew it was possible due to the
  use of the return path in LAN networks.

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Considerations of the forward path

• One output feeding multiple inputs
• Known output level
• Well defined signals in terms of frequency
  mapping, bandwidth, and modulation
• The test location is the same as the adustment
  location.
• Most importantly, each amplifier receives its
  input from only one source

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Return Path Considerations

• No standard frequency mapping
• Todays return signals vary widely in modulation
  techniques, bandwidth, and power levels
• Long Loop AGC
• The measurement location is different from the
  adjustment location.
• Return amplifiers receive signals from several
  different sources!
• ADDED REVENUE STREAMS
• High Speed Internet Services
• Telephony
• VOD

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Optimizing the node, the first step

• Find out total input power rating for return
  lasers
• Inject one CW carrier at that level into the node
• Set the X or headend reference value

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Finding the X value in the headend
3.5 dB
To return receiving equipment
X
4.1 dB
2.9dB
3.7 dB
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Headend combining and splitting
Other Return Services
CMTS
Telephony
Set top converter
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Why do we need Unity Gain?
32/26
30/24
31/25
29/23
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22
22
22
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23
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If Unity Gain is not observed distortions build
up quickly!
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Unity Gain in the forward path
R
R
Each amplifier compensates for the loss in the
cable and passives before the amplifier under
test. The system is aligned so that the levels
at each green arrow are exactly the same.


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Constant outputs in the return path?
R
R
Return Equip.
If the return amplifiers were balanced with
constant outputs, the levels would vary widely
by the time they got back to the headend. This
is due to return amplifiers having several
inputs.

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Constant inputs instead of constant outputs for
Unity Gain
R
R
Return Equip.
In the return path, amplifier outputs are
balanced for a constant input to the next
amplifier upstream. This maintains Unity Gain,
but makes the measurement more difficult.

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Different Balancing Techniques

• The 2 person method
• The Round Robin method
• The sweep method

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The 2 person method
R
R
Return Equip.
The reference signal is injected at the input to
the reverse amp farthest from the headend.
Measurements are taken at the input to the next
amplifier upstream.

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Flaws of the 2 person method

• Very labor intensive
• Communication problems between the 2 technicians
• This does not test the frequency response of the
  return path!

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The Round Robin method
R
R
Return Equip.
Camera

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Problems with this method

• High equipment costs
• TV and signal generator in the bucket at the same
  time, I wonder if anything ever gets dropped?
• No measurement device control
• Dedicated 6 MHz channel needed in the upstream
• Still no frequency response measurement!

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How does reverse sweep work?
R
R
Return Equip.
RF in

RF out
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Advantages of return sweep

• Still only one person
• Align forward and reverse with the same stop at
  the amplifier
• No cumbersome equipment in the field or the
  headend
• Minimum use of bandwidth for test equipment
• Control over the measurements
• We are aligning the entire spectrum in both
  directions, not just 2 carriers!

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Is Sweep really necessary?1 Carrier Balancing
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Is sweep really necessary?4 carrier balancing
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Properly Aligned Return Path No Ingress Problem
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5 steps to set up your return path correctly

• Know your equipment
• Determine reference levels
• Determine reference points
• Optimize return lasers
• Sweep coaxial portion of the plant

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Know your equipment, know your system

• Block diagrams of amplifiers, nodes, receivers,
  etc.
• Test Equipment
• What are the return design levels
• What are the injection points

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How is a reference level determined?
From trunk return
58 dBmv modem output 26db tap 2 dB drop loss 9 dB
directional coupler
21dBmV at the reference point
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Picking the reference point

• At the input to the reverse hybrid
• At the input to the port

KEY Be Consistent
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Advantages of using the port reference point

• Higher modem outputs for better C/I
• Simpler to calculate injection levels

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Injection levels
20 dB test points
21 dBmV at the reference points means an
injection level of 41 dBmV
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Normalizing the sweep trace
R
R
Return Equip.
RF in

RF out
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Finishing the process!
R
R
Return Equip.
RF in

RF out
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Upstream Impairments

• Common Path Distortion
• Ingress

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Ingress

• Ingress is a combination of random and periodic
  noise and discrete signals leaking into the cable
• Usually generated in the customers home
• Excessive ingress can cause the return laser to
  clip
• Ingress from anywhere affects the entire system

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Upstream Ingress

• Ingress refers to interference typically found on
  (but not limited to) the return path. Most
  ingress comes from the drops.
• Some sweep systems detect ingress on their return
  sweep data frequency and broadcast the display
  data to the field on the forward data carrier for
  display.

Return Path without Ingress
Return Path with Ingress
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You cant get there from here
The problem could be here
To CMTS Receive Port
Spare Splitter Leg
Optical Receiver
Fiber Node
Optical Receiver
or here
Optical Receiver
or here
CoaxDist.Network
The actual Call might be here
or the problem could be anywhere in these three
nodes.
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Ingress is always worse from the lower value taps!
45 dBmV output at design frequency
Forward Losses
5dB
7dB
9dB
26
20
14
4
2dB
1.5dB
1dB
Reverse Losses
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How can we minimize ingress?

• Quality cable and connectors
• Good installation practices
• Better than mandatory leakage program
• Taps with equalizers

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Common Path Distortion

• A series of beats easily seen in the return
  spectrum at repetitive 6 MHz spacings
• Ingress does not cause repeatable patterns
• Usually caused by corrosion at a dissimilar
  metals interface acting as a diode
• Actually caused by the forward carriers and also
  increases distortions in the forward path
• The higher in level the forward carrier levels
  are at the source of the problem, the worse CPD
  will be

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Corrosion Diode Effect

• Crystallization occurs and the corrosion creates
  thousands of small diodes between the two metals
• Diodes are non-linear devices that can act as
  frequency mixers in a CATV plant

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Frequency Mixing
Mixing two frequencies (F1 F2) will yield four
results
55.25 MHz 61.25 MHz 116.50 MHz 6.00 MHz
F1 F2 F1 F2 F2 F1
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Common Path Distortion

• A corroded connection causes mixing
• The resulting impedance mismatch also causes
  reflections
• The mixing products are reflected right back into
  the return amplifier.

Corroded Connection
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Downstream Signals
Difference frequencies reflected upstream
(6, 12, 18, 24)
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CPD in 6 MHz Intervals

• Because the channels in the forward system are 6
  Mhz apart, the sum and difference frequencies
  occurr at 6 MHz intervals as well.

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CPD common sources

• Loose or over-tightened seizure screws
• Loose hold down screws on modules and circuit
  boards
• Feed through connectors
• Loose and corroded terminators on taps
• Bad line terminators on high value tap
• Anything that allows moisture to enter a device

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CPD troubleshooting tips

• When return sweeping, set up sweep from 5-50 MHz
• Check distortions on the forward path above your
  highest channel
• Once the feeder leg is found, troubleshoot from
  the termination and work back toward the amplifier

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Troubleshooting Goals

• To be able to localize problems without taking
  the system down!
• Identify problems from the field without a trip
  to the hub or headend first.

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Troubleshooting Hints

• Know your test equipment
• Know your amplifier configurations
• Establish what a good return path looks like from
  different test points
• Low pass filter on the spectrum analyzer
• AC blocking seizure screw probe
• Tap jumpers
• Return Path problems are relative to where they
  are being measured

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Troubleshooting

• Isolate the node
• Isolate the feeder
• Isolate the tap

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Isolating the Node
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3010R Return Spectrum
Current Node
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Typical Node RF Block Diagram
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Typical RF Bridging Amplifier Block Diagram
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Troubleshooting to the tap
Tap Isolation 20dB down
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Thank You!
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References

• National Cable Television Associations
  Recommended Practices for Measurements on Cable
  Television Systems, 2nd Edition, October 1997
  Supplement on Upstream Transport Issues.
• Broadband Return Systems for HFC Cable TV
  Networks, by Donald Raskin and Dean Stoneback
• Return Path Level Selection, Set Up, and
  Alignment Procedure, Motorola 1997
• Modern Cable Television Technology, by Walter
  Cicora, James Farmer and David Large

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More References

• Mystified by Return Path Activation? Get your
  Upstream Fiber Links Aligned, by Ron Hranac,
  Communications Technology, March 2000
• Seek Balance in All Things A Look at Unity
  Gain in the Upstream Coax Plant, by Ron Hranac,
  Communications Technology, June 2000
• A Primer on Common Path Distortion, by Nick
  Romanick, Communications Technology, April 2001

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Telephony over an Broadband HFC system
Central Office with LDS
Powering Node
Combining Networks
Phone B
several T-1s
HDT
DMC
Phone A
NIU
OPC Computer
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NIUs, NIDs, Voice Ports, whatever you want to
call em
To outside plant
RF Diplexer
RF Telephony Tuner
RF Modem
POTS Interface Board
Power Supply
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Inputs to the reverse amplifiers
Forward Losses
5dB
7dB
9dB
26
20
14
4
2dB
1.5dB
1dB
Reverse Losses
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Which reference point should we use?
From trunk return
21 dBmV at the reference point
10 dBmV at the reference point
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Reference points at a line extender
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21 dBmV at the port reference point, modem
operates at 58 dBmV
10dBmV at the hybrid reference point,
modem operates at 48 dBmV
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Input Level Matrix

Type of Hardware
Laser Hub
Line Ext
Trunk Amp
Bridger
Sweep Input Level
10 dBmV
10 dBmV
10 dBmV
10 dBmV
Internal Coupling Loss
4 dB
1 dB
5 dB
13 dB
Test Point Loss
30 dB
20 dB
20 dB
20 dB
Total Insertion Point Loss
34 dB
21 dB
25 dB
33 dB
Source Level
44 dBmV
31 dBmV
35 dBmV
43 dBmV
Source Level Sweep Input Level IP
10 34 44 dBmV
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Fast Intermittents

• 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