National Instruments Toolkit

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 Vrms, its power spectrum units are in V2rms. If you divide by the

impedance, you get W2rms.

•Spectrum or spectral density—The PSD is the power spectrum

divided by the frequency resolution. PSD units are usually V2rms/Hz or

W2rms/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:

Vrms Hz()⁄

dBm rms2

------------

⎝⎠

⎛⎞

10 10 XVrms[]()

2Window size

FFT size

--------------------------------

1000 mW

---------

ENBW df Hz[]impedance Ω[]××

-----------------------------------------------------------------------------------

⎝⎠

⎜⎟

⎛⎞

log×=