Trilithic 720 DSP, Operation Manual

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Page IV-190
720 DSP Operation Manual
Business Services Meter
Using CM Sweep
The current method used to check
for gain and flatness in the return
CATV band is to insert test carriers
(red dots) between live signals (blue/
green traces), receive these signals
at the headend sweep receiver, and
basically connect the dots, producing
a frequency flatness trace, as
shown in the image to the right. The
problem with this method is available
bandwidth. As more channels are
inserted into the upstream, the
available bandwidth to insert test carriers is diminished.
An alternate method of testing the
upstream is to process information
between the CMTS and the
meter’s cable modem, in order to
present a bar graph representation
of the upstream carriers. This
allows the technician the ability to
analyze the upstream for tilt and
gain without the added expense of
a headend sweep receiver.
The image shown on the right is
a return band (upstream) sweep
using DOCSIS carriers.
Each bar represents the upstream channel compared to the nominal value expected at the
CMTS. Assuming the CMTS is looking for zero, the first channel is indicating that it is about
5 dBmV lower than the CMTS is expecting and the fourth channel is about 3 dBmV higher
than the CMTS nominal value.
The bar graphs above also incorporate the pre-equalization information from the CMTS
which gives the technician an indication of the in-channel response for each upstream
channel. This can aid the technician in quickly evaluating the upstream channel response.
There are two agile markers that allow you to check for gain (tied to the first marker) and tilt
(which is the difference between the two markers) both across the upstream band and in-
channel. Peak , Valley , and P/V are referenced between the markers.