Tektronix 2715, Руководство пользователя

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Spectrum Analyzer Menus
2715 User Manual
6–51
To determine the occupied bandwidth, the 2715 first sums the signal power in all
512 frequency cells of the displayed spectrum, ignoring any contributions more
than 40 dB below the signal peak. (This is the total displayed signal power, P
t
.)
It then sums the power in the cells starting at the left-hand edge of the screen
until the accumulated power equals or exceeds the following:
P
t
1
2
(1
n
100
)
Pt = total signal power
n = percentage of signal power within the occupied BW
It then performs a similar calculation starting at the right-hand screen edge. That
is, it finds the frequency cells, both above and below the signal, beyond which
half the power not-in-the-occupied bandwidth resides. The frequency difference
between the upper and lower cells is the n% occupied bandwidth.
For instance, if you specify the 90% occupied bandwidth, the 2715 first
computes the total signal power. Next, it sums the signal power in the cells
starting at the left-hand edge until it accumulates 5% of the total. (The percent
signal power not-in-the-occupied bandwidth is (100% – 90%) = 10%, and half of
it is below, and half above, the occupied BW). See Figure 6–11.
5% Power 90% Power 5% Power
f
l
–f
h
=90% Occupied BW
f
l
f
h
Figure 6–11: Illustration of Occupied BW Measurement
If the accumulated power does not precisely equal half the power not-in-the-oc-
cupied bandwidth (the usual case), the 2715 linearly interpolates between
frequency cells to arrive at better frequency estimates.