2-14-2002Page 1

Return Path Testing Seminar Presented by

Sunrise Telecom Broadband …a step ahead

2-14-2002Page 2

Introduction to Return Path Testing

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.

2-14-2002Page 3

Noise and Ingress Funneling

Noise and ingress from anywhere in the node can effect all users on that node

tap tap

tap tap

tap tap

Noise or Ingress

Return Modem Signal

Return Modem Signal

Node

Noise Ingress and Modem

Signals Combine

To Headend

2-14-2002Page 4

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.

tap tap

tap tap

tap tap

Noise or Ingress

Return Modem Signal

Return Modem Signal

Node

To Headend

Check at various points in the system to locate source

of ingress or noise

2-14-2002Page 5

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

2-14-2002Page 6

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

2-14-2002Page 7

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.

Current Sweep

Max Hold Trace

2-14-2002Page 8

Zero Span

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.

FrequencyFrequencyAm

plit

ude

Am

plit

ude

Am

plit

ude

Am

plit

ude

TimeTime

Frequency Domain Time Domain

2-14-2002Page 9

Zero Span

By parking on a frequency the analyzer will now display any amplitude variations over time.

Zero span displays the instantaneous amplitude of the carrier over the time of the display.

Time

Modem Bursts

Display In Zero Span

2-14-2002Page 10

Zero Span

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

2-14-2002Page 11

Zero Span Markers

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

1st Modem Burst

Modem Level (Mkr 1)

Desired to Undesired

Ratio

Noise Level (Mkr 2)

2nd Modem Burst

2-14-2002Page 12

Zero Span Trigger Threshold

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.

2-14-2002Page 13

Zero Span Time Per Division

Since the zero span display is in the time domain, the horizontal axis is calibrated in 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 get the type of display you are looking for.

2ms/div

200s/div

2-14-2002Page 14

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 bandwidth such as 300 kHz.

2 MHz Resolution Bandwidth

300kHz Resolution Bandwidth

2 MHz RBW required to capture entire bandwidth

2-14-2002Page 15

Return Path Testing Conclusions

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.