Digital Testing
DigiPoints, Volume 2
Page 11.1
Student Workbook
DigiPoints Volume 2
Student Workbook
Module 11 – Digital Testing
Summary
In this module, students learn engineering and operational information about Digital
Testing.
Module Objectives
Upon successful completion of this module, the student will be able to:
•
•
•
•
•
•
Discuss the differences between modulation formats for the digital signal.
Identify key system impairment measurements.
State some examples of the use of spectrum analyzers and QAM monitors for digital
testing.
Explain the role of constellation and eye diagrams in digital signal impairment
analysis.
Interpret constellation diagrams to facilitate system diagnostics.
Describe the role of the technician in performing digital testing at the headend and in
the field.
Prerequisites
Read DigiPoints, Volume 2, Chapter 11.
As you complete this module, you should be filling in answers and definitions for the
items found on the following pages.
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Digital Testing
DigiPoints, Volume 2
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Module 11 – Digital Testing
Take notes on the following pages as your instructor covers the topics in this session. For
some topics, key points are identified. You should add to them with supplementary
information from the class discussions. For other topics, you will need to fill in key
points as requested in the workbook.
Constructing meaningful notes in this way he lps you to understand the subject and can
serve as a valuable future reference.
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DigiPoints, Volume 2
Page 11.3
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What Can Be Tested?
Digital equipment must have two key features:
High Reliability
High Modularity
Identify the main reason(s) for both requirements.
System reliability is ensured by preventive maintenance programs and built- in
monitoring.
Monitoring can detect unintended changes to data content as well as equipment failure.
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DigiPoints, Volume 2
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Types of Network Tests and Measurements
Core Network Tests
Core networks deliver MPEG streams from content providers to service providers.
Core networks may be carrying services in addition to digital cable.
Responsibility for core maintenance and test — a specialized discipline — is with the
carrier.
Provide an example of a core network.
Provide an example of an access network.
Access Network Tests
Access networks move signals from the headend to the subscriber.
Adequate characterization of network performance requires three types of measurement.
Spectrum tests
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DigiPoints, Volume 2
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Signal Modulation tests
Signal Integrity tests
Provide an example of each type of measurement.
Spectrum tests
Signal Modulation tests
Signal Integrity tests
Each of the above may require the following measurement categories.
In-channel characteristics
Interference due to other channels
Signal impairments
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DigiPoints, Volume 2
Page 11.6
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Spectrum Tests
A spectrum analyzer provides a visual representation of the signal amplitude in the
frequency domain (i.e., a “plot” of amplitude versus frequency for each “component” of
the signal under test.
To standardize methods of system test, spectrum analyzer measurements are often
performed at a common resolution and filtering.
Subsequent results are used to set a system performance baseline which, when met, will
determine with reasonable confidence that a transmitted signal will arrive at the receiver
within accepted levels of signal quality.
List common system measurements (DigiPoints provides 14 examples).
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Interpretation of the measured results requires an understanding of the differences
between an analog and digital signal.
List the key differences between the modulation of analog and digital signal.
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DigiPoints, Volume 2
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Workbook Exercise #1
Complete Workbook Exercise #1. Figure 11-2 from the text provides a plot of an analog
and digital signal trace as viewed on a spectrum analyzer. (Note: the settings on the left
side are for a digital signal.)
Figure 11-2
Analog and Digital Signal Traces
Source: Tektronix
Identify the following.
The analog signal trace
The digital signal trace
The three signal peaks in the analog signal trace
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DigiPoints, Volume 2
Page 11.9
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In-Channel Characteristics
Channel Mean Power
Channel Mean Power is a measurement of the average power of the modulated signal.
Maintaining correct carrier power levels on digital carrier throughout the system is
crucial for optimum system performance.
On a digital carrier, the power is spread out more evenly over time and frequency
compared to an analog carrier.
To get an accurate representation of the power you must measure the total power across
the frequency band, not just at one narrow frequency the way an analog SLM measures
level.
Figure 11-3
Averaged Power Measurement Result
Spectrum Analyzer
Signal Level Meter
Source: Tektronix
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DigiPoints, Volume 2
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Occupied Bandwidth
This measurement is taken to avoid interference with adjacent channels.
For 64 QAM, 99% of the signal power exists within 4.84 MHz of the 6 MHz channel.
Typically, the assigned bandwidth of digital channel occurs at 30-40 dBc and not at the
99% power point.
Figure 11- 4
99% Occupied Power Bandwidth
vs. Assigned Bandwidth
99% of Transmitted Power
Assigned Channel Width
Source: Tektronix
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Peak Power
This measurement is taken to avoid signal compression and intermodulation interference.
Due to the random nature of the signal peaks, measurement requires the storage of a
series of results that will display both:
The distribution of amplitude variations
The peak-to-average ratio which can be used to determine system transmission linearity
along the cable path
As a rule of thumb for QAM and VSB, the peak power exceeds the Channel Mean Power
by as much as 6-10 dB.
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Carrier Power
This measurement is taken for comparison with analog carrier levels. It is used with
noise power levels to calculate the Energy-per-bit-to-Noise ratio.
Carrier power should be the same as the total modulated signal power.
To calculate the carrier power manually, the tester examines the spectrum analyzer trace
to determine the power level at the carrier frequency and the resolution bandwidth.
That power level must be adjusted by the log ratio of occupied bandwidth to measuring
(resolution) bandwidth, according to the formula:
Pa = Pm + 10 log (Ws / Wr)
Where,
Pa = Actual carrier power
Pm = Measured power
Ws = Signal bandwidth
Wr = Spectrum analyzer measurement bandwidth
Identify the main disadvantages with this manual method.
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Interference due to other channels
This measurement determines how much signal power from one-channel spills over into
an adjacent channel.
Two possible sources of interference are:
Adjacent digital channel
Adjacent analog channel
Figure 11-5
Adjacent Channel Spurious Interference
Source: Tektronix
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Describe the characteristics of adjacent digital channel interference.
Describe the characteristics of adjacent analog channel interference.
Desired to Undesired (D/U) is defined as the ratio of the level of channel power on an
active channel to that on the same channel when it is out of service.
This measurement is very similar to the analog signal-to-noise ratio (SNR), but includes
both noise and distortion in the undesired component.
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DigiPoints, Volume 2
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Signal Impairment
Noise Power tests (NPR) are a variation of the D/U test performed when all channels are
out of service.
A random phase, multitone signal or software simulation in the analyzer can be used to
sweep across the system bandwidth.
Figure 11-7
NPR Test
Undesired interference
For actual system
Source: Tektronix
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DigiPoints, Volume 2
Page 11.16
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Modulation Tests Using Displays of the
Constellation Diagram
QAM analyzers display the constellation diagram and can identify the placement of
individual symbol landings.
Decision boundaries exist within the constellation such that:
If the signal falls within the decision boundary, the correct data will be received.
If because of noise or other interference, the symbol falls outside the decision boundary,
the data will be in error.
Unless error detection and correction is employed, the error will result in video
impairment.
Figure 11- 8
Constellation Diagram
Q
Transmitted Symbols
(shown as dots) are
the individual symbol
landings
I
Inter-symbol
Interference Zones
Decision Boundaries
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Figure 11-9 and 11-10 represent normal operating signals for 64 and 256 QAM.
Figure 11-9
64 QAM Constellation Diagram
Source: HukkEngineering
Figure 11-10
256 QAM Constellation Diagram
Source: Hukk Engineering
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DigiPoints, Volume 2
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Workbook Exercise 2
Break into small groups of four to six people, and complete this exercise as a group.
Figures 11-11 through 11-16 are constellation diagrams that indicate fault conditions.
Label the fault and discuss possible causes within your group.
Figure 11-11
Q
Rectangular, rather
than square, pattern
I
Figure 11-12
Q
I
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Figure 11-13
Q
I
Figure 11-14
Q
I
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Figure 11-15
Q
I
Figure 11-16
Q
I
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Workbook Exercise # 3
Write a short description of the following Digital Testing keywords.
Access Network –
Core Network –
Decision boundary –
I signal –
Q signal –
Symbol –
Symbol landing –
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Digital Testing
DigiPoints, Volume 2
Page 11.22
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Signal Impairment Measurements
List the key points of each of the following indicators of signal impairment.
Energy-per-Bit/Noise
1.
2.
Error Vector Magnitude
1.
2.
3.
Figure 11-17
Error Vector Magnitude
Target Symbol
Q
Error Vector
Transmitted
Symbol
I
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DigiPoints, Volume 2
Page 11.23
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Modulation Error Ratio
1.
2.
3.
Bit Error Rate
1.
2.
3.
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DigiPoints, Volume 2
Page 11.24
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Appendix: Constellation and Eye Diagrams
Two common formats of digital modulation are QPSK and QAM.
Both are examples of multibit modulation that employ amplitude and phase to modulate
the carrier.
Within the QPSK format, the carrier has constant frequency but is processed as two
channels, I and Q, operating in quadrature.
I is termed In-Phase.
Q is termed Quadrature.
The digital data is carried equally by both channels.
The channels are then combined to provide a single RF output that has four possible
phases corresponding to four possible data states where:
Two bits of data can be represented as 22 = four phases.
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Two bits of data are termed a symbol and can be transmitted in a 1 Hz bandwidth.
At the receiver, a pair of coherent AM demodulators, fed by signals that are 90 degrees
out of phase with each other, can separate the I and Q signals as part of the demodulation
process.
Complete the table below which relates the carrier phase to the bit combination.
Phase
Bit Combination
45 degrees
135 degrees
225 degrees
315 degrees
QAM permits the carrier to be both amplitude and phase modulated.
64 QAM provides eight amplitude levels on each I, Q channel:
8x8 = 64 QAM
16x16 = 256 QAM
Why is 64 QAM more robust in the presence of noise than 256 QAM?
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Why is QPSK more robust in the presence of noise in comparison to 64 QAM?
To facilitate analysis of the modulated carrier, two visual tools have evolved: eye and
constellation diagrams.
An eye diagram displays the I or Q waveform with respect to time where multiple
waveforms are overlaid which take the shape of an eye.
Real world imperfections will provide signal variance “closing the eye” such that the
receiver or test equipment’s ability to differentiate between a “1” or a “0” decreases.
Constellation diagrams provide a two-dimensional representation of the I channel with
respect to the Q channel where:
The amplitude of the I is plotted against the amplitude of Q.
A noise free signal plot would appear as a rectangular pattern of n dots.
A more practical signal plot appears as a cluster of n dots.
Analysis of the cluster shape provides information on the modulation quality.
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DigiPoints, Volume 2
Page 11.27
Student Workbook
Workbook Exercise # 4
Testing Your Knowledge
1. What are the three types of tests that can be performed on access networks?
2. What are the categories for the measurements that are obtained from the tests in your
answer to question 1, and what is the purpose of each category of test?
3. What is the difference between an analog TV signal and a digital TV signal as viewed
on a spectrum analyzer, and what causes the difference?
4. In multibit modulation, what is the symbol? How many bits are contained in the
symbol associated with QPSK? With 64 QAM? With 256 QAM?
5. In a digital signal, what is the theoretical relationship between carrier power and the
total modulated signal power?
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6. What are the four in-channel characteristic measurements and why are they taken?
7. Name three measurements of interference due to other channels and indicate why
they are taken.
8. Name six modulation fault conditions that can be detected from a constellation
diagram.
9. What is MER, and what does it measure?
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DigiPoints, Volume 2
Page 11.29
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Laboratory Exercise #1
Scope:
This laboratory exercise is intended to allow the student to participate in and better
understand the practice of digital testing at the headend and in the field.
Overview:
Digital pictures do not show signal impairment until it is too late (i.e., a picture that
appears of an acceptable quality could be approximately 0.5 dB away from failing
completely). This is known as the cliff-edge scenario and makes the measurement of
digital parameters critical to determine the system margin.
This is in contrast to the more traditional analog environment where signal impairments
are clearly visible and it was easy to assess the performance of the cable system by
knowing system levels and monitoring the received signal.
The Digital Signal Analyzer (or QAM Monitor) is a CATV maintenance and
troubleshooting tool that supports in-service measurements of 64 and 256 QAMmodulated digital signals and traditional analog signals.
The analyzer can present the measured information in one of three formats:
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Constellation Display
Spectrum Display
Signal Level Meter
Constellation Display
A constellation display is an X/Y plot of the I and Q channels and provides a graphic
representation of the QAM modulated signal. Refer to VA 11.7A for an example of a 64
QAM constellation diagram.
Ideally each of the 64 or 256 symbols should display a clean dot, indicating a perfect
QAM signal. The size and shape of the build up of these dots indicates the amount and
type of impairment such as noise, interference, phase noise or gain.
Constellation displays can also indicate the presence of in-band spurs or hum.
Since all of these impairments can cause bit errors, this display is a valuable tool for
identifying and troubleshooting these problems.
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DigiPoints, Volume 2
Page 11.30
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Spectrum Display
The Spectrum Mode allows spectral display of any 6 MHz channel and allows for the
identification of the type of channel, either analog or digital, and for viewing any noise or
spurs in the channel.
Signal Level Meter
The Signal Level Meter Mode measures both analog and digital carrier levels.
In analog mode, both the video and audio carrier levels are indicated as well as the delta
audio/video.
On digital channels, the average power over the 6 MHz channel is measured and
displayed.
Laboratory Set-Up
The Lab is written with the assumption that we have access to a QAM analyzer to display
constellation, spectrum and signal level diagrams. The CR1200-QAM (manufactured by
Hukk Engineering) is suitable for monitoring both 64 and 256 QAM.
For the purpose of the lab we will create a “mini- headend” which includes, video source,
digitizer, MPEG-2 encoder, QAM modulator. Refer to Figure 1.
Second Source X can be used to inject an adjacent channel or a noise source.
Digitizer
VIDEO SOURCE
+
MPEG-2
ENCODER
QAM
MODULATOR
(64/256)
SECOND
SOURCE "X"
BASEBAND
NOISE SOURCE
Fig 1 – Laboratory Set-Up - Digital Channel
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1. QAM Constellation Measurement
Application: Digital Testing at the headend
Outline: Connect the equipment as per Figure 1.
Connect the QAM monitor to the cable network signal. Pad signal to monitor if
necessary.
Verify that signal constellation measured as per Figure 11-9 for 64 QAM (Figure 11-10
for 256 QAM).
2. Determine carrier average power
Application: Digital Testing at the headend
Outline: Configure QAM monitor for spectrum display.
Determine carrier average power.
Outline: Configure QAM monitor for signal level meter.
Determine carrier average power.
Verify that both methods of measurement are in agreement or can be correlated.
3. Determine signal MER
Application: Digital Testing at the headend
Outline: The Modulation Error Ratio display on the CR1200 is analogous to the Signalto-Noise measurement made in analog systems.
Poor MER is an early indicator of channel impairment.
Determine signal MER.
Add baseband noise to the video signal source and determine the MER.
View constellation diagram of “noisy” data and compare against results from the first
task above and Figures 11-9 and 11-10.
4. Determine signal BER
Application: Digital Testing at the headend
Outline: The Bit Error Rate measures how often an error occurs in a given amount of
data. BER is useful for measuring long term system performance.
Determine the signal BER.
Add noise to the baseband video source.
Determine the signal BER.
Compare constellation diagrams.
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Page 11.32
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5. Gain Compression
Application: Digital Testing at the headend
Outline: Increase the signal to generate gain compression. If both the I and Q channels
are affected, the outer points will be pulled in while the inner points are not affected.
Verify that measured result corresponds to Figure 11-16.
6. Coherent Interference
Application: Digital Testing at the headend
Outline: The constellation clusters take on a circular shape as a result of coherent
interference. Examples of coherent interference include CTB, CSO, ingress and spurs.
Using second source “X”, inject a cont inuous CW spur in channel of QAM signal.
Determine impact of spur on constellation.
7. Adjacent channel interference
Application: Digital Testing at the headend
Outline: Configure the analyzer for spectrum display. Center display on carrier channel,
with both upper and lower adjacent channels displayed.
Determine if the digital signal spills over into the adjacent bandwidth.
8. Review Figures 11-11 to 11-16 which provide examples of constellation
diagrams with known fault conditions.
In discussion with your leader, identify possible component failures that would generate
such constellation diagrams.
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