X-Stream Operator’s Manual
WM-OM-E Rev I 403
nbpw Narrow Band Power
Definition Provides a measurement of the power at a specific frequency for a
waveform.
Description nbpw is the magnitude of the Discrete Fourier Transform (DFT) computed
on a waveform at a specific frequency. nbpw calculates one bin of a DFT
centered at the frequency provided. The bin width is 0.105% of the
frequency selected if the waveform trace on the DDA is 960*
(1/frequency) or more in length (i.e. the trace is equal to or longer than
960 cycles of a waveform at the selected frequency). Otherwise, the bin
width is:
100 / integer[trace length/(1/frequency)] %,
where integer[ ] designates discarding any fractional portions in the result.
Thus, if the waveform trace is 48.5 times longer than 1/frequency then the
bin width will be:
100/48 = 2.1% of the selected frequency.
A Blackman-Harris window is applied to the input data to minimize
leakage effects. The net result is that nbpw will provide excellent results
even if frequency changes occur due to spindle speed variations. If the
actual frequency differs from the specified frequency, and the bin width is
0.105% (minimum resolution bandwidth), the resulting power will be
reduced from the actual as in this table:
Frequency Difference dB Reduction
0.03% 0.3 dB
0.06% 1.1 dB
0.1% 3 dB
If the bin width is greater than 0.105%, the frequency difference for which
a specified dB reduction will occur will scale proportionally to the bin
width/0.105.
nbpw results are presented in dBm.
Parameter Settings Selecting this parameter accesses a frequency setting dialog.
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