CATV Measurements
2715 User Manual
5–41
H
Resolution bandwidth: 300 kHz
H
Video filter: Off
H
Acquisition mode: Peak
H
Reference level: Signal peak is at top of screen
H
Vertical mode: Linear
H
Span: Zero Span
H
Sweep speed: 20 ms/div
A time domain (ZERO SPAN) waveform is acquired. This waveform is
converted to the frequency domain using a Fast Fourier Transform (FFT)
calculation so that the percent hum for the power line fundamental and 1
st
harmonic can be measured. The hum values are calculated from the FFT result as
follows.
4
100
Power Line Frequency Component
Visual Carrier Component
The factor of four (4) comes from the following two sources:
H
The two-sided nature of the FFT requires that the apparent levels of each
harmonic be doubled
H
The result must be doubled again to obtain the peak-to-peak harmonic levels
since the FFT finds only the peak value of each harmonic component
The hum data values are shown in the two readout lines as follows:
ZZ.Z% TOTAL HUM
XX.X% @ F
0
YY.Y% @ F
1
where F
0
is the power line fundamental (50 Hz or 60 Hz)
and F
1
is the power line 1
st
harmonic (100 Hz or 120 Hz).
The time domain data is displayed in waveform B.
The following procedure shows how to make a HUM/LFD measurement:
1. Press [CATV/APPL] to call up the CATV MEASUREMENTS menu. See
Figure 5–1.
2. Press [6] to select the HUM/LFD measurement menu.
Enable HUM/LFD
Measurement
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