CATV Measurements
2715 User Manual
5–49
CTB and CSO Measurements
The methods to make these measurements are: Interactive, Automatic,
Automatic with Pause-for-Carrier-Off, Single Sweep, and (for CSO only)
Continuous. Also, some of the CSO measurement methods can be performed on
an in-service basis. Interactive mode helps you search for beats. The automatic
modes let you measure beats when you already know where they occur. For the
Automatic, Automatic with Pause-For-Carrier-Off, and Single-Sweep modes,
CSO is usually measured at offsets of
±
0.75 MHz and at
±
1.25 MHz for NTSC
(National Television Systems Committee) systems. CTB is measured at zero
offset. A CTB may also be viewed between channels 4 and 5 for a standard
channel plan.
Regardless of the mode selected, the first step is to measure the peak level of the
visual carrier in the current channel. When operated in Continuous mode, the
carrier frequency measured is that specified in the Setup menu. This measure-
ment is the reference level that all subsequent measurements are referred to. Up
to 5 beats are measured. The results are presented in terms of dBc relative to the
carrier level measured at the beginning.
When the CTB and CSO measurement is done on a digital channel, the average
channel power is measured. The Interactive mode and Automatic with Pause
modes are used to perform the measurement. This measurement is not available
in the in-service mode.
NOTE
. The CTB or CSO test result may be marked with an asterisk (
*
). This
indicates when a beat signal is within 2 dB of the 2715’s noise level. Under these
conditions, the displayed dBc value may be lower than the actual value (that is,
the beats may have a smaller amplitude than the displayed value).
The gated CSO test result may also have 1000, 2000, or 3000 added to its
magnitude. This indicates that certain events occurred during the measurement
which may cause an inaccurate result. If so, the inaccuracy is on the conserva-
tive side; the actual CSO will be weaker than the measured value.
If 1000 is added to the result, the instrument detected that one or more of the
quiet lines found at the start of the test were no longer quiet at the end of the
test. This can occur only on gated measurements.
If 2000 is added to the result, the instrument increased the signal level at the 1st
mixer input above optimum to ensure adequate signal level for accurately-timed
gating control. For CSO measurements, this normally occurs on channels that
are only a few dB weaker than the general system level.
If 3000 is added to the result, both of the above events occurred.
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