NI Spectral Measurements Toolkit User Guide
30
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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 2
N
,
the result contains twice as much energy in the 2
N
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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×
⎝
⎠
⎜
⎟
⎜
⎟
⎜
⎟
⎛
⎞
log
×
=
