Transcript QAM Digital Measurements Seminar

Return Path Testing Seminar
Presented by
Sunrise Telecom Broadband …a step
Page 1
ahead
2-14-2002
Introduction to Return Path Testing
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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 and the zero span display of a spectrum
analyzer are very useful tools for analyzing the return path
and the signals carried on it.
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Noise and Ingress Funneling
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Noise and ingress from anywhere in the node can effect
Return
all users on that node
Modem
Signal
tap
tap
tap
tap
tap
tap
To
Headend
Node
Noise Ingress
and Modem
Signals
Combine
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Return
Modem
Signal
Noise or
Ingress
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Using a Spectrum Display to Track Ingress
and Noise

Use a spectrum analyzer display to track the source of
noise and ingress in the system.
Return
Modem
Signal
tap
tap
tap
tap
tap
tap
To
Headend
Node
Check at various points in
the system to locate source
of ingress or noise
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Return
Modem
Signal
Noise or
Ingress
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Limitations of Spectrum Displays for
Catching Fast Transients.
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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
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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
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Max Hold Function
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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 Trace
Current Sweep
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Zero Span
Amplitude

Zero Span mode on a spectrum analyzer is a very useful
tool for viewing and measuring fast transient signals or
ingress.
Zero Span mode stops the sweep of the spectrum analyzer
and parks it at the center frequency changing the display
from the frequency domain to the time domain.
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Frequency
Frequency Domain
Amplitude

Time
Time Domain
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Zero Span
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By parking on a
frequency the analyzer
will now display any
amplitude variations
Time
over time.
Zero span displays the
instantaneous
amplitude of the carrier
over the time of the
display.
Modem
Bursts
Display
In Zero
Span
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Zero Span
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Amplitude variations displayed will include cable modem
bursts and intermittent ingress.
Because the center frequency is always tuned to the carrier
and doesn’t sweep, extremely fast transients can be viewed.
Cable Modem
Signals
Noise Floor
viewable
between modem
bursts
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Zero Span Markers
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Markers in Zero span allow you to measure the level of
individual modems and the desired to undesired ratio.
Noise and ingress can be viewed at the carrier frequency
between modem bursts
Marker 1
Marker 2
2nd Modem
Burst
1st Modem Burst
Modem Level
(Mkr 1)
Noise Level (Mkr 2)
Desired to
Undesired
Ratio
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Zero Span Trigger Threshold
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Since return path modem signals appear at random times it
is necessary to trigger the display on a modem signals
rising edge to catch it at the correct time. Trigger level is
adjustable to match the level of the modem signal.
Trigger
Threshold
adjusted to
about 50%
of burst
amplitude.
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Zero Span Time Per Division
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Since the zero span display is
in the time domain, the
horizontal axis is calibrated in
2ms/div
time/division.
Selecting a longer time per
division will show a longer
snapshot of the modem traffic.
Longer times will not display
the resolution in time so
adjust the time/division to get200ms/div
the type of display you are
looking for.
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Effects of Resolution Bandwidth on
Zero Span
Since the analyzer is parked at the
center frequency and doesn’t scan,
only signals within the bandwidth of
the resolution bandwidth filter will
be displayed.
 To accurately display wide band
signals such as cable modem return
path signals a wide resolution
bandwidth filter such as 2 MHz is
required to accurately display the
signal.
 When viewing narrower signals
such as noise and ingress it may be
better to use a narrower resolution
Page 14 bandwidth such as 300 kHz.
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2 MHz Resolution
Bandwidth
300kHz
Resolution
Bandwidth
2 MHz RBW required to
capture entire bandwidth
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Return Path Testing Conclusions
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Testing on the return path is much different than on the
forward path due to noise and ingress funneling and the
bursty nature of return path signals.
The spectrum analyzer display is a very useful tool for
tracking down ingress and noise in the field.
The zero span display of a spectrum analyzer stops the
frequency sweep and turns the display into a time domain
display of the signals in the frequency band of the
resolution bandwidth filter.
The zero span display of a spectrum analyzer allows you to
see fast transients events such as modem bursts and ingress
as well as the noise in between the events.
2-14-2002