Computes the averaged FFT spectrum of time signal. This VI returns the FFT results as real and imaginary parts. Wire data to the time signal input to determine the polymorphic instance to use or manually select the instance.


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Inputs/Outputs

  • cbool.png restart averaging (F) —

    restart averaging specifies whether the VI restarts the selected averaging process. If restart averaging is TRUE, the VI restarts the selected averaging process. If restart averaging is FALSE, the VI does not restart the selected averaging process. The default is FALSE.

    When you call this VI for the first time, the averaging process restarts automatically. A typical case when you should restart averaging is when a major input change occurs in the middle of the averaging process.

  • cmsdt.png time signal —

    time signal is the input time-domain waveform.

  • cu32.png window —

    window (Hanning) is the time-domain window to apply to the time signal. The default window is Hanning.

    0Rectangle
    1Hanning (default)
    2Hamming
    3Blackman-Harris
    4Exact Blackman
    5Blackman
    6Flat Top
    74 Term B-Harris
    87 Term B-Harris
    9Low Sidelobe
    11Blackman Nutall
    30Triangle
    31Bartlett-Hanning
    32Bohman
    33Parzen
    34Welch
    60Kaiser
    61Dolph-Chebyshev
    62Gaussian
  • cerrcodeclst.png error in (no error) —

    error in describes error conditions that occur before this node runs. This input provides standard error in functionality.

  • cnclst.png averaging parameters —

    averaging parameters is a cluster that defines how this VI computes the averaging. The specifications of the parameters include the type of averaging, the type of weighting, and the number of averages.

  • cenum.png averaging mode —

    averaging mode specifies the averaging mode.

    0
    No averaging
    (default)
    1
    Vector averaging
    2
    RMS averaging
    3
    Peak hold
  • cenum.png weighting mode —

    weighting mode specifies the weighting mode for RMS and vector averaging.

    0
    Linear
    1
    Exponential
    (default)
  • cu32.png number of averages —

    number of averages specifies the number of averages used for RMS and vector averaging. If weighting mode is exponential, the averaging process is continuous. If weighting mode is linear, the averaging process stops after this VI computes the selected number of averages.

  • cdbl.png window parameter —

    window parameter specifies the beta parameter for a Kaiser window, the standard deviation for a Gaussian window, and the ratio, s, of the main lobe to the side lobe for a Dolph-Chebyshev window. If window is any other window, this VI ignores this input.

    The default value of window parameter is NaN, which sets beta to 0 for a Kaiser window, the standard deviation to 0.2 for a Gaussian window, and s to 60 for a Dolph-Chebyshev window.

  • ibool.png averaging done —

    averaging done returns TRUE when averages completed is greater than or equal to the number of averages specified in averaging parameters. Otherwise, averaging done returns FALSE. averaging done is always TRUE if the selected averaging mode is No averaging.

  • icclst.png real part —

    real part returns the real part of the averaged FFT spectrum and the frequency scale.

  • idbl.png f0 —

    f0 returns the start frequency, in hertz, of the spectrum.

  • idbl.png df —

    df returns the frequency resolution, in hertz, of the spectrum.

  • i1ddbl.png real part —

    real part is the real part of the averaged FFT spectrum.

    If the input signal is in volts (V), real part has units of volts-rms (Vrms). If the input signal is not in volts, real part has units of the input signal unit-rms.

  • icclst.png imaginary part —

    imaginary part returns the imaginary part of the averaged FFT spectrum and the frequency scale.

  • idbl.png f0 —

    f0 returns the start frequency, in hertz, of the spectrum.

  • idbl.png df —

    df returns the frequency resolution, in hertz, of the spectrum.

  • i1ddbl.png imaginary part —

    imaginary part is the imaginary part of the averaged FFT spectrum.

  • idbl.png averages completed —

    averages completed returns the number of averages completed by the VI at that time.

  • ierrcodeclst.png error out —

    error out contains error information. This output provides standard error out functionality.

    • The FFT Spectrum (Real-IM) VI completes the following steps to compute real part and imaginary part:

    1. Computes the FFT of time signal.
    2. Averages the current FFT spectrum of time signal with the FFT spectra computed by the VI since the last time the averaging process was reset.
    3. Returns the real and imaginary parts of the averaged spectrum.

    The single-channel version of this VI can perform single-channel measurements in both one-shot mode, meaning a single call, and continuous mode, meaning multiple calls with history. The single-channel version can perform multichannel measurements only in one-shot mode. If you want to make multichannel measurements in continuous mode, use the multichannel version of this VI.

    The single-channel version of this VI maintains internal state information for a single channel only. Calling the single-channel version to process another channel without using the restart averaging control to clear the history results in an unexpected behavior of this VI. The unexpected behavior results from the VI passing the internal state information from one channel to another.

    Note The single-channel version of this VI is intended primarily for continuous processing of a single channel. Do not generalize this behavior to the multichannel case and use the single-channel version in a For Loop to continuously process multiple channels by indexing an array of waveforms.

    Refer to the following LabVIEW project for examples of using a multichannel version of a VI like the FFT Spectrum (Real-IM) VI in a loop:

    • labview\examples\Signal Processing\Waveform Measurements\Waveform Measurements.lvproj

    Related Information

    Output Units for FFT-Based VIs