Return Path Maintenance and Troubleshooting

Return Path Alignment
Node Return Laser set
up.
Two-Way HFC CATV System Overview
HEADEND OUTPUT
DOWNSTREAM
LASER
NODE
AMPLIFIER
OPTICAL FIBER
OPTICAL FIBER
COAXIAL CABLE
UPSTREAM
OPTICAL RECEIVER
TO UPSTREAM DATA
RECEIVERS
AMPLIFIER
(cont.)
Forward Path Unity Gain
IN
+10 dBmV
(0 dB input atten.)
OUT
+32 dBmV
IN
+16 dBmV
(6 dB input atten.)
OUT
+32 dBmV
452.9 meters
288.3 meters
2 dB
22 dB @ 750 MHz
8 dB
14 dB @ 750 MHz
11 dB @ 750 MHz
226.5 meters
IN
+13 dBmV
(3 dB input atten.)
OUT
+32 dBmV
OUT
+32 dBmV
Reverse Path Unity Gain
OUT
IN
+24 dBmV
+20 dBmV
(11 dB output atten.)
IN
+20 dBmV
OUT
IN
+25 dBmV
+20 dBmV
(10 dB output atten.)
452.9 meters
288.3 meters
2 dB
4 dB @ 30 MHz
8 dB
3 dB @ 30 MHz
2 dB @ 30 MHz
226.5 meters
OUT
+30 dBmV
(5 dB output atten.)
IN
+20 dBmV
CATV Forward Distribution Network Design
Values shown are at 750 MHz
Amplifier
downstream output:
+40 dBmV
+3 dBmV
+2 dBmV
2.7 dB
+1 dBmV
Feeder cable: 0.500 PIII, 2.16 dB/100 ft
Drop cable: 6-series, 5.65 dB/100 ft
17
2.7 dB
-1 dBmV
125 ft
14
2.7 dB
-3 dBmV
8
11.1 dB
20
3.3 dB
125 ft
11.1 dB
2.7 dB
2.0 dB
125 ft
11.1 dB
11.1 dB
2.7 dB
23
1.8 dB
125 ft
11.1 dB
26
Modem
input:
1.6 dB
125 ft
11.1 dB
1.4 dB
-3 dBmV
CATV Return Distribution Network Design
Values shown are at 30 MHz
Amplifier upstream
input:
+18 dBmV
Modem TX: +49 dBmV
+47 dBmV
0.5 dB
+45 dBmV
Feeder cable: 0.500 PIII, 0.4 dB/100 ft
Drop cable: 6-series, 1.22 dB/100 ft
17
0.5 dB
+44 dBmV
125 ft
14
0.5 dB
+43 dBmV
8
5 dB
0.5 dB
20
1.9 dB
125 ft
5 dB
5 dB
0.5 dB
23
1.3 dB
125 ft
5 dB
26
1.2 dB
125 ft
5 dB
0.8 dB
125 ft
5 dB
0.6 dB
+39 dBmV
Setting up the Return Path
Finding the “X” Level
 Determining the Return Transmitter
“Window”
 Padding the Transmitter
 Return Receiver Setup
 Distribution out of the Return
Receiver
 Padding the inputs to the Headend
Equipment

Setting Upstream Signal Levels
X level
 The easiest way to set upstream signal levels is to
establish what is called the X level.
• This is a headend upstream signal level that is the
result of providing the proper level at the input to
the last reverse amplifier (the first amplifier or
node out of the headend).
 To establish the X level, go to the first downstream
amplifier or node location out of the headend.
• Here you should inject a signal into that location’s
reverse amplifier module input at a level known to
be correct.
• This will result in a signal at the headend that is
measured and defined as the X level.
 Assuming your system was designed for unity gain
operation, when you go to the next amplifier location
and inject the proper amplitude test signal there, the
resulting signal at the headend will be the same as
the original X level.
Setting the Transmitter “Window”

RF input levels into a return laser
determine the CNR of the return
path.
• Higher input – better CNR
• Lower input – worse CNR



Too much level and the laser ‘clips’.
Too little level and the noise
performance is inadequate
Must find a balance, or, “set the
window” the return laser must
operate in
• Not only with one carrier but all the
energy that is in the return path.
• The return laser does not see only one
or two carriers it ‘sees’ the all of the
energy (carriers) that in on the return
path that is sent to it.
Energy in the Return Path


What does your return path look like?
The return laser ‘sees’ all the energy in the return path.
• The energy is the sum of all the power of all the carriers in the
spectrum from about 1MHz to 42 MHz.
• The more energy that is put into the laser the closer you are to
clipping the laser.
• A clean return path allows you to operate your system more
effectively.
• The type of return laser you use has an associated window of
operation
• We can show you the window of operation of each laser using a NPR
(Noise Power Ratio) curve.
Ingress Changes over Time
Node x Instant
Looks Pretty Good
Node x Overnight
Oh No!
What is NPR?



NPR = Noise Power Ratio
Is means of easily characterizing an optical
link’s linearity and noise contribution
NPR and CNR are related, but not the same…
7dB Optical Budget Composed of
a Spool of Single Mode Fiber +
Connectors and Splices
Composite Noise Power
Adjusted in 1dBmV Steps
Transmitter
Variable
Attenuator
Noise
Source
Notch Filter
Receiver
Laser
Driver
Spectrum
Analyzer
Rx
Noise
Noise
Power / Hz
37MHz
NPR  10 Log (
PS ( Hz )
)
PN ( Hz )
Signal
Power / Hz
FP and DFB NPR Curves

FP and DFB NPR curves at room temperature.
Non-Linear Response
(Clipping)
Standard DFB & FP TX
Noise Power Ratio (NPR) Performance
with 7 dB Optical Link
Linear Response
55
50
38 dB CNR
40
35
30
Room Temp Std DFB
Room Temp Std FP
25
20
-70
-65
-60
-55
-50
-45
-40
-35
Input Power per Hz (dBmV/Hz)
Total RF
Input
Power
CarriertoNoise*
NPR (dB)
45
-30
-25
-20
-15
Signal Clipping

RF ingress and impulse noise may cause signal
clipping
• Can affect Composite power into return laser


Excessive signals from in-home devices such as
pay-per-view converters also may cause signal
clipping
Clipping occurs in upstream amplifiers and fiber
optics equipment
• FP Upstream lasers most susceptible


Energy that can cause clipping found mostly from
5 MHz to 15 MHz range
Signals at all other frequencies are affected by
cross-compression
• Cross-compression affects all upstream frequencies
• Can reduce data throughput
(TCP/IP controlled resend)
Adding Carriers to the Return Path
Per Carrier Power vs. Composite Power
21dBmv
Power into
Transmitter: 21 dBmV
FSK
Modulation
21dBmv
FSK
Modulation
Power into
Transmitter: 24 dBmV
Per Carrier Power vs. Composite Power
21dBmv
FSK
Modulation
Power into
Transmitter: 24 dBmV
21dBmv
Power into
Transmitter: 27 dBmV
FSK
Modulation
Per Carrier Power vs. Composite Power



As you add more carriers to the
return path the composite
power to the laser increases.
To maintain a specific amount
of composite power into the
transmitter the carrier power
must be reduced.
When modulation schemes are
changed the composite power
into the transmitter changes.
• The higher the order of
modulation the more energy the
channel contains.
FSK
BPSK
QPSK
QAM-16
QAM-32
QAM-64
Video
Rel Power
-22.6
-19.0
-19.0
-12.1
-8.2
-6.0
0.0
Changing Modulation Type
21dBmv
FSK
Modulation
Power into
Transmitter: 24 dBmV
21dBmv
QAM 16
Modulation
Power into
Transmitter: 34 dBmV
Changing Modulation Type
21dBmv
QAM 16
Modulation
Power into
Transmitter: 34 dBmV
21dBmv
QAM 64
Modulation
Power into
Transmitter: 40 dBmV
Different Services require Different CNR

HSD
• 16 QAM

STB (VOD)
• QPSK

Telemetry
• FSK

Business Services
• QPSK to 16 QAM

ModulationType Required CNR

Multiple services on the
return path with different
types of modulation
schemes will require
allocation of bandwidth
and amplitudes.
• Can be engineered.
• Requires differential
padding in Headend
• Required CNR for various modulation
schemes to achieve 10E-8 BER
 BPSK: 12dB
 QPSK: 15dB
 16QAM: 22dB
 64QAM: 28 dB
 NOTE: DOCSIS calls out 25dB min CNR
Determining Power Levels

Power per Hz:
• Power per Hz = total power - 10log(total bandwidth in Hz)

Channel power from power per Hz
• Channel power = power per Hz + 10log(channel bandwidth
in Hz)
Determining Digital Power Levels

Example: Calculate allocated channel power for a 2 MHz wide
QPSK digitally modulated signal carried in the reverse path of the
previous example.
• Channel power = power per Hz + 10log(channel bandwidth in Hz)
• Channel power = -30.44 + 10log(2,000,000)
• Channel power = +32.57 dBmV
RF Level Changes at Transmitter
Room Temp
Standard FP TX
Noise Power Ratio (NPR) Performance
9MHz Noise Bandwidth with 1:1 HE Combining
- 40 F
+ 140 F
45
NPR (dB)
40
35
8dB
30
25
20
-65
11.5dB
-60
PLaser – GAmp + Pad = Pin
-55
-50
-45
-40
-35
-30
-25
-20
Input Power per Hz (dBmV/Hz)
32 dBmV @ 9MHz
32 - 14 + 7 = 25 dBmV
Pad = 7dB

As pad values are changed the input to the Headend devices change
•
•

Return Amp Setup
Power Level = 18dBmV
1:1 Ratio
Must change headend attenuation (setup) to maintain the ‘X’ level.
Do not change pad value or increase RF level into the node once the Laser
operational window has been set.
Return RX Setup



On analog returns from the node the less optical power into a
receiver the less RF you will have on the output.
The RF levels on the output of the return receivers should be set
with internal or external RF attenuation such that with the X level
that is placed into the forward test point on the node X level will
exist on the output of all receivers.
To much optical power can cause intermodulation in the receiver
• Typical maximum input -3 dBm
• Use optical attenuators on extremely short paths or where to much
optical power exists into a receiver

To little optical power can cause CNR problems with that return
path.
• If combined with other return receiver outputs can create noise issues
on more paths
Return RX Setup

Rule of Thumb (company specific):
• Do not optically attenuate the return path so all the optical
inputs are the same as the lowest.
• The lower the optical input power the lower the CNR of the
receiver.
• Attenuate RF internally or externally of the device

Must have enough level so that the CMTS or other devices
receiving the signals from the return path operate
acceptably.
• There can be excessive passive loss from the output of the
optical receiver to the terminating device.
 8-way splitter/combiner – 10.2 dB typical
 4-way splitter/combiner – 6.8 dB typical
• Typical input into terminating device.
 CMTS – 0 dBmV
Changes to the Return Network
ANY CHANGES TO THE RETURN PATH
FROM THE SUBSRIBER TO THE
HEADEND CAN AFECT IT’S
PERFORMANCE


Planned
• Segmentation of Return
 Changes in HE or Node
Un-Planned
• Bad tap
• Optronics Failure
• Ingress
• Technician – Laser RF input level changes in
the field
Questions