HP 8591C, User Manual, Page: 277 / 373

I
The hum test measures the percentage of amplitude modulation for low
frequency disturbances (hum) by measuring the peak-to-peak variation for
total hum. The Fast Fourier Transform (FFT) is also used to display the low
frequency spectrum around the carrier. This information can be used to
identify the source of hum.
Total hum is measured by reading the peak-to-peak variation in the visual
signal level caused by low frequency disturbances (hum or repetitive
transients).
The hum test automatically the analyzer for measuring with or
without video modulation on the carrier. This is necessary because analyzer
settings used to match the
NCTA
cannot be used
when video modulation is present. Other analyzer settings can be used to
compensate for the presence of video modulation, then the measurement can
be made.
When video modulation is on, the video bandwidth is set to 1 MHz to allow
the analyzer to measure only the values of the sync tips. See Figure 8-6.
When the video bandwidth is set to 1 MHz,
noise is also present.
This noise is digitally minimized, then several peak-to-peak measurements
are taken and averaged. Using the average of these measurements gives
the best consistency with
oscilloscope measurements. However, the
effects of noise cannot be completely eliminated and the results reported have
slightly more uncertainty (usually less than 1 percent larger) than when video
modulation is off. The reverse, however, is not true. That is, when adding
modulation to a carrier wave signal, results will increase, not decrease. For
example, if the results are less than 3 percent with modulation on, FCC rule
is If a channel does exceed 3 percent, it may still pass
by testing again without video modulation.
When video modulation is not present, the test sets the analyzer to the
conditions set forth in the
NCTA Recommended Practices. See
Figure 8-7.
A setting of zero span with a sweep time of 5 acts as a signal level
meter and oscilloscope display, and a video bandwidth of 1 functions as a
1 low pass Elter. Several peak-to-peak measurements are taken, then the
average measurement is reported.
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