National Instruments NI Spectral Measurements Toolkit User Manual
Page 30
NI Spectral Measurements Toolkit User Guide
30
ni.com
Spectrum scaling options are combinations of the following options:
•
RMS or peak—An FFT returns an amplitude spectrum scaled such
that a frequency bin represents the RMS value of a sine wave at that
frequency. A bin can also represent the peak value if you scale the
spectrum by
.
•
Amplitude or power—The power spectrum is the squared magnitude
of the amplitude spectrum. For example, if an amplitude spectrum has
units of V
rms
, its power spectrum units are in V
2
rms
. If you divide by the
impedance, you get W
2
rms
.
•
Spectrum or spectral density—The PSD is the power spectrum
divided by the frequency resolution. PSD units are usually V
2
rms
/Hz or
W
2
rms
/Hz. You can also obtain an amplitude spectral density by
choosing units such as
.
Connect the spectral info output parameter of the Zoom FFT VIs to the
spectral info input parameter of the SMT Spectrum Unit Conversion VI.
This parameter includes the following subparameters:
•
window determines the ENBW of the window you use. The ENBW
affects the spectral density calculations because of the spectral leakage
effect of windowing in the frequency domain.
•
The ratio of window size and FFT size is a value the SMT Spectrum
Unit Conversion VI uses to correct any difference between the number
of frequency bins in the spectrum and the number of points in the
time-domain signal. The correction ensures that you can preserve
the energy of the original signal. For example, if you zero-pad a
time-domain signal of length N, or window size N, to a length of 2N,
the result contains twice as much energy in the 2N frequency bins as is
in the time-domain signal. Given the two sizes, you can compensate for
this effect.
For example, you can use the SMT Spectrum Unit Conversion VI to
perform PSD measurements with units dBm/Hz on a signal. Set units to
dBm, peak scaling to RMS, psd? to
TRUE
, and impedance to the system
impedance. PSD is calculated using the following formula:
2
V
rms
Hz
(
)
⁄
dBm
rms
2
Hz
------------
⎝
⎠
⎛
⎞
10
10 X Vrms
[
]
(
)
2
Window size
FFT size
--------------------------------
×
1000
mW
W
---------
ENBW df Hz
[
] impedance Ω
[ ]
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⎟
⎜
⎟
⎜
⎟
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