Distortion Measurements Express VI

Owning Palette: Signal Analysis Express VIs

Requires: Full Development System

Performs distortion measurements on a signal, such as tone analysis, total harmonic distortion (THD), and signal in noise and distortion (SINAD).

Dialog Box Options
Block Diagram Inputs
Block Diagram Outputs
 Add to the block diagram  Find on the palette

Dialog Box Options

ParameterDescription
DistortionContains the following options:
  • SINAD (dB)—Calculates the measured signal in noise and distortion (SINAD). SINAD is defined as the dB of the ratio of the RMS energy of Signals to the RMS energy of Signals less the energy in the fundamental. To compute the THD Plus Noise in dB, negate the SINAD.
  • Total harmonic distortion—Computes the measured total harmonic distortion up to and including the highest harmonic. THD is defined as the ratio of the RMS sum of the harmonics to the amplitude of the fundamental tone. To compute THD as a percentage, multiply it by 100.
  • Specific harmonic level—Returns the harmonic you specify.
  • Harmonic number (fundamental = 1)—Specifies the harmonic to measure. This option is available only when you select Specific harmonic level.
  • Stop search at Nyquist—Specifies to include only frequencies less than the Nyquist frequency, or half the sampling rate, in the harmonic search. This option is available only when you select Total harmonic distortion or Specific harmonic level. When you remove the checkmark from the Stop search at Nyquist checkbox, this VI continues searching the frequency domain beyond the Nyquist frequency by assuming that the higher frequency components have aliased according to the following equation:

    aliased f = Fs – (f modulo Fs)

    where

    Fs = 1/dt = sampling rate

  • Highest harmonic—Controls the highest harmonic, including the fundamental tone, to use for the harmonic analysis. For example, for third harmonic analysis, set Highest harmonic to 3 to measure the fundamental, second, and third harmonic. This option is available only when you select Total harmonic distortion or Specific harmonic level.
Search FrequencyContains the following options:
  • Search for fundamental frequency—Controls the frequency domain search area, which is the center frequency and width, to use for finding the fundamental tone frequency of the signal.
    • Approximate frequency—Center frequency to use in the frequency domain search for the fundamental tone frequency. The default is 0. If you set Approximate frequency to –1, this Express VI uses the tone with the highest amplitude as the fundamental tone. This option is available only when you place a checkmark in the Search for fundamental frequency checkbox.
    • Search (+/- % of approx. freq.)—Frequency width, as a percentage of the sampling rate, to use for the frequency domain search for the fundamental tone frequency. The default is 5. This option is available only when you place a checkmark in the Search for fundamental frequency checkbox.
ResultsDisplays the measurements you configured this Express VI to perform and the calculated values of those measurements. You can click any measurement listed in the Measurement column, and the corresponding value or plot appears in the Result Preview graph.
Input SignalDisplays the input signal. If you wire data to the Express VI and run it, Input Signal displays real data. If you close and reopen the Express VI, Input Signal displays sample data until you run the Express VI again.
Result PreviewDisplays a preview of the measurement. The Result Preview plot indicates the value of the selected measurement with a dotted line. If you wire data to the Express VI and run the VI, Result Preview displays real data. If you close and reopen the Express VI, Result Preview displays sample data until you run the VI again. If the cutoff frequency values are invalid, Result Preview does not display valid data.

Block Diagram Inputs

ParameterDescription
SignalsContains the input signal or signals.
error inDescribes error conditions that occur before this node runs.

Block Diagram Outputs

ParameterDescription
THDReturns the measured total harmonic distortion up to and including the highest harmonic. To compute THD as a percentage, you must multiply it by 100.
Specific HarmonicReturns the harmonic you specify in Specific harmonic level.
SINAD (dB)Returns the measured signal in noise and distortion (SINAD). SINAD is defined as the dB of the ratio of the RMS energy of Signals to the RMS energy of Signals less the energy in the fundamental. To compute the THD Plus Noise in dB, negate the SINAD.
error outContains error information. This output provides standard error out functionality.

This Express VI operates similarly to the following VIs and functions:

Harmonic Distortion Analyzer
SINAD Analyzer

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