Transcript Return Path Issues and Answers

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 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
55-319MHz
Com21
2 Way
RF
splitter
23
20
430MHz
2 Way
RF splitter
HCX comController
600MHz
17
Public switch
CMTS
14
Router
Headend Combining
RX
RX
RX
Sweep
Modem
RX
RX
RX
Phone
Analyzer
RX
RX
RX
Headend Combining
RX
RX
RX
Sweep
Modem
RX
RX
RX
Phone
Analyzer
RX
RX
RX
Energy Accumulation
Return Path Signal
Funnel Effect Cont..
Funnel Effect Cont..
CPU
Monitoring
Device
Combiner
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
Average case
Return Rx
Return TX
Return Noise Floor
System behavior
Noise funneling (amplifiers + optics)
Return Rx
Return TX
Return Noise Floor
System behavior
Return Rx
Return TX
noise from modem
System behavior
Laser Clip
Return Rx
Return TX
Ingress 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 Components
2
Head-End RX
Head-End RX
TX
3
Head-End RX
Head-End RX
Combiner
1
CMTS Demod
5
4
7
6
Optical return path link
Node
Headend
Forward TX
Rx
Return Rx
Return TX
• Optimum level
– input level too low ==> low thermal and RIN CNR
– input level too high ==> high Intermodulation noise
==> low CNR
CNR
BER
optimum level
input level
optimum level
input level
Node-Hub Return Link
Alignment in the Field (1)
Person 1
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
System Sweep
Sweep Transmitter
Transmitter 3SR
3SR
Stealth Sweep
help
FILE
1
abc
4
jkl
7
stu
def
2
ghi
3
FREQ
status
AUTO
5
mno
6
vwx
9
pqr
CHAN
yz
ENTER
alpha
SETUP
8
x
.
PRINT
LEVEL
TILT
SCAN
SWEEP
C/N
HUM
MOD
SPECT
space
0
+/-
CLEAR
light
FCN
10
40
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
+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
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
Based on Channel Bandwidth
(42-5)/1.6=23 channels
10*log(23ch)=13.6 dB
22-13.6= 8.4dB per Channel
(22dBmV value 2dBmV below QAM Clip)
Node adjustment
Test point sticker level
is level for video carriers
=> for digital, target is
TP level is 8dB
Specified level into forward TP is 39dBmV
Corresponding input level
is 19dBmV
(20dB)
NRT Field Alignment
(From the GNA Installation manual)
+39 dBmV
+8
+5
-5
0
H
L
-20dB
+19
NRT
+8 dB
(Input to TP @min atten)
NRIK
-5 dB
NRIC
Diplex
-6 dB
-3 dB
• 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
Optical Transmitter
Combining
Network
Node
Optical Receiver
TP
Out
System
System Sweep
Sweep Transmitter
Transmitter 3SR
3SR
Stealth Sweep
help
FILE
1
abc
4
jkl
7
stu
2
def
ghi
3
FREQ
status
AUTO
5
mno
6
vwx
9
pqr
CHAN
yz
ENTER
alpha
SETUP
8
x
.
space
0
+/-
CLEAR
light
FCN
Optical Receiver
PRINT
LEVEL
TILT
SCAN
SWEEP
C/N
HUM
MOD
SPECT
3ST
Reverse Sweep Displayed on 3SRV
3SRV
Reverse Sweep Reference
Reverse Sweep
Return Path Requirements
Signal Levels
Passive 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 single10Log N
• C/N single = Input + 59 –
N.F.
What return amplifier inputs
are required?
•
•
•
•
•
•
•
•
•
•
-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)
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
18dBmV
Ref
Note Reference levels at
Headend and retain for rest
of amp chain
(Start with longest link)
49dBmV
Return Amplifier Set-Up
Headend
Ref
15dBmV
L
“X”
“X”dB
Output
Equaliser
(per map
Design)
“X”dB
Output
Attenuator
(per map
design)
Level applied to
return amp input
(Take into account
The test point loss and
the
Amplifier embedding
loss)
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-Up
Headend
15dBmV
Ref
Set Attenuator
to get correct
signal level in
Head End
In-Home Signal Losses
Example of losses at 40MHz
Tap
150'
RG-6
-2.1 dB
-3.5 dB
50'
-0.8 dB
50'
-0.8 dB
Drop
-2.1 dB
Splitter
-3.5 dB
RG59
-0.8 dB
Splitter
-3.5 dB
RG59
-0.8 dB
=============
-3.5 dB
Total
-10.7 dB
We will use -10dB as the typical in-house and drop loss.
RF Plant Passive Losses
7 dBmV
embedding loss +15 dBmV
Relative to Return Amp Input
+23 dBmV
+25 dBmV
+28 dBmV
at amp input
-5
-1.0
Cable Losses
@870 MHz
26
@40 MHz
+15 dBmV
at amp input
A= Closest to node
High tap Value
B= Farthest from node
low tap value
+22
dBmV
needed
at
Input to
Housing
-7
-2.0
23
+48 dBmV
into tap port
-11
-3.0
20
17
+45 dBmV
into tap port
-10 dB
internal and
drop loss
-10 dB
internal and
drop loss
+58 dBmV
Modem output
+55 dBmV
Modem output
Plant Actives - Type Amps
Relative to Return Amp. Input
+42dBmV
H
H
0
L
L
+22 dBmV
H
0
L
5-LER-91
H
0
+15 dBmV
-2 dB
L
-7 dB
Network Amplifier
Plant Actives - LE
Relative to Return Amp. Input
+47dBmV
H
L
0
H
L
+17
dBmV
5-LER-91
-2 dB
+15
dBmV
-2 dB
+37dBmV
Line Extender
Return Set Up relative to Return
Amplifier Input
-20dB TP @ +42 dBmV
-30dB TP @ +52 dBmV
+35 dBmV
H
L
+35 dBmV
0
2 Pad
5 EQ.
+15 dBmV
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
H
L
0
H
L
0
H
L
H
L
+22 dBmV
9 Pad
5 EQ.
H
L
+15 dBmV
+47 dBmV
+ 35
dBmV
H
0 L
23
+17 dBmV
40 dBmV
9 Pad
5 EQ.
+15 dBmV
0
H
L
0
H
L
0
H
L
+22 dBmV
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
Common Mode Distortion
• 6 MHz Beats
• Cause
• Location
Common Mode Distortion
REF 6.0 dBmV
ATTEN 10 dB
MKR 8.90 MHz
-15.93 dBmV
PEAK
LOG
5 dB/
0 Hz
RES BW 300 KHz
VBW 100 KHz
40.00 MH
SWP 20 msec
Common Path Distortion
6MHz
FSK
Interfering Beat
Common Path Distortion
25dB C/N
@ Status Monitoring
Frequency
Modem Return
Spectrum
Common Problems
Seizure
Coax
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
Return
source of noise and ingress in the system.
Modem
Signal
To
Headend
Node
tap
tap
tap
tap
tap
Check at various points in
the system to locate source
of ingress or noise
Noise or
Ingress
tap
Return
Modem
Signal
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
Max Hold Trace
Current Sweep
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.