CATV Measurements
2715 User Manual
5–19
The successful application of gated measurements, therefore, depends on good
quality video signals. In particular, you should avoid attempting to make
in-service measurements on channels carrying video from low quality sources
such as consumer grade VCRs and camcorders. In addition to the noise and
flatness problems, most low quality video sources produce head switching
glitches or other disturbances which tend to disrupt the timing of the 2715’s gate
operation. Such disruptions may cause measurement errors ranging from subtle
to dramatic.
When performing in-service tests, the instrument routine checks for quiet lines at
lines 10 through 20 and 272 through 282 on NTSC systems or at lines 6 through
20 and 318 through 332 on PAL systems. If the only quiet lines available are
beyond this range, then you must specify the quiet line in the channel table,
using the Cable TV RF Measurements Software for Windows provided with your
instrument. In this case, the in-service test is run on the specified line, even
though it is outside the normal range, but there is no verification that the line is
still quiet after the test.
The 2715 firmware includes an algorithm which automatically sets the RF
attenuator and RF preamplifier states to values based on a measurement of the
signal levels on the system throughout the frequency range. This is done for the
same reason that the overall operating level in a CATV system needs to be
controlled — to make a suitable compromise between noise and distortion.
In some instances, this may cause an unexpected result. The gating hardware
used for some of the in-service measurements needs a strong signal to perform
reliably. In a nonflat system, weaker signals on the system may fail to meet this
requirement because the optimum RF attenuator and preamplifier settings are
based on the levels of the stronger signals present, causing the weaker signals to
have excessively low levels at the 1st mixer of the 2715. In this case, the 2715
automatically increases the 1st mixer input level (by the minimum amount
required). However, increasing the 1st mixer input level may cause the measure-
ment to be corrupted by instrument-generated distortion products. Although this
technique will sometimes give an inaccurate test result, the error is in the
conservative direction. That is, if the test result reports that the CATV system
passes, it most likely has passed by a greater margin than indicated by the
measurement result. Note that whenever the 1st mixer input level has to be
increased, the test result is flagged by adding 2000 to its magnitude (see
individual test descriptions for more details).
If a measurement is flagged by adding 2000 to its magnitude, you must decide
whether or not to accept the measurement. If the numerical result is acceptable,
no further action is necessary. If you want a more accurate measurement, you
should consider using preselection in front of the 2715 (either use a CATV
preselector or make the measurement through a set top converter, if applicable).
Alternatively, use a nongated measurement, which will not need to make the 1st
mixer level readjustment.
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