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The following table displays the output units and scale factors that the LabVIEW FFT-based VIs use.

VI |
Non-DC Unit(k>0) |
DC Unit(k=0) |
Sided |
Non-DC Scale Factor |
DC Scale Factor |
Phase Unit |
---|---|---|---|---|---|---|

FFT | V | V | Double | N/2 | N | radians |

Power Spectrum | V_{rms}^{2} |
V^{2} |
Double | 1/2 | 1 | — |

Auto Power Spectrum | V_{rms}^{2} |
V^{2} |
Single | 1 | 1 | — |

Amplitude and Phase Spectrum | V_{rms} |
V | Single | 1 | 1 | radians |

FFT Power Spectrum and PSD (power spectrum measurement) | V_{rms}^{2} |
V^{2} |
Single | 1 | 1 | — |

FFT Power Spectrum and PSD (power spectral density measurement) | V_{rms}^{2}/Hz |
V^{2}/Hz |
Single | 1 | 1 | — |

FFT Spectrum (Mag-Phase) | V_{rms} |
V | Single | 1 | 1 | radians/degrees |

FFT Spectrum (Real-Imag) | V_{rms} |
V | Single | 1 | 1 | radians/degrees |

The **VI** column of the previous table displays the VI name. The **Non-DC Unit** column displays the output unit for non-DC components where the bin number, or array index, is nonzero. The **DC Unit** column displays the output unit for the DC or offset component where the bin number is zero, indicating the first element of the spectrum array. k is the spectral bin number defined by the equation f/f = f/(fs/n). V is Volts, and V_{rms} is Volts root-mean-squared, or Volts rms. VIs with a **dB On** control map the output unit onto a dB scale by computing 10*log(unit^{2}) if the output unit is normally squared, or 20*log(unit) for non-squared units. These VIs reference all dB values to 1.0.

The **Sided** column of the previous table displays whether the VI computes the double-sided or single-sided spectrum. A double-sided spectrum has non-DC component magnitudes that the VI halves between the positive and negative frequency bins, corresponding to the first and second halves of the double-sided spectrum array respectively. All double-sided VIs associate a non-unity scale factor with the results, and the **Non-DC Scale Factor** and **DC Scale Factor** columns list these values. N is the input time-domain signal size, or the number of samples. A single-sided spectrum returns only the positive frequency components, or the components in the first half of the double-sided spectrum. The single-sided spectrum also compensates for all scale factors these VIs associate with the double-sided spectrum.

The Power Spectrum VI, Auto Power Spectrum VI, FFT Power Spectrum VI, and FFT Power Spectral Density VI compute power as a real quantity and thus return no phase information. The FFT VI, Amplitude and Phase Spectrum VI, FFT Spectrum (Mag-Phase) VI, and FFT Spectrum (Real-Imag) VI compute the real and imaginary parts of the spectrum phase. These VIs reference all phase values to a cosine so that an FFT component at bin *k* with an amplitude of *A* volts and radians can be represented as the time-domain signal:

where *i* = 0, 1, ... *N*–1.

The **Phase Unit** column of the previous table displays the resulting phase units. A **Phase Unit** of radians/degrees indicates that you can use the **convert to degree** control to change the phase units from radians to degrees.

Refer to the FFT and Power Spectrums Units VI in the labview\examples\measure\maxmpl.llb for an example converting units using the FFT-based VIs.