National Instruments NI Spectral Measurements Toolkit User Manual page 30

2
rms
⎛ ⎞
dBm 10
----------- -
=
⎝ ⎠
Hz
NI Spectral Measurements Toolkit User Guide
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
impedance, you get W
• Spectrum or spectral density—The PSD is the power spectrum
divided by the frequency resolution. PSD units are usually V
2
W /Hz. You can also obtain an amplitude spectral density by
rms
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
impedance. PSD is calculated using the following formula:
⎛
⎜
2
× ⎜ ( [ ] )
log 10 X Vrms
⎜
⎝
2
.
, its power spectrum units are in V
2
.
rms
⁄ (
V Hz
rms
TRUE
Window size
× ×
------------------------------- - ---------------------------------------------------------------------------------- -
FFT size ENBW
30
2
. If you divide by the
rms
)
.
, and impedance to the system
mW
1000
-------- -
W
× [ ] impedance Ω ×
df Hz
2
/Hz or
rms
⎞
⎟
⎟
[ ]
⎟
⎠
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