2.4.17. Octave Plots

Octave Plots display the RMS amplitudes of the outputs from a bank of band pass filters. The center frequencies and transfer characteristics of the band filters are selected according to the requirements of the IEC 61260 standard. The bottom band will always be the standard 15.625Hz center frequency one (1000Hz/64), the available top frequency band will be determined based on the sample rate of the channel. These plots:

  • Require the selection of the following items on the Plot Creation tab before adding them to the plot grid:

    • One or more primary data channels.

  • Extend the Plot Menu with an additional sub-menu described below.

  • Do not add extra pages to the Plot Setup dialog.

  • Provide no additional user interaction mechanisms beyond the ‘standard’ ones described in the Plot Interactions section.

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Example Octave Plot and Menu

The Plot Menu extension displayed for octave plots provides the following functionality:

  • dB scaling: Toggles the Y axis scaling between signal RMS amplitude and decibel modes.
  • 0 dB level: Available in dB scaled mode only. Sets the data level that corresponds to 0 dB.
  • Bands per Octave: The [1 .. 6] menu choices below this entry allow configuring the number of frequency bands (i.e. band filters) created for each octave of the frequency range.
  • Weighting: The [none, A .. D] menu choices below this entry allow configuring the weighting curve being applied to the calculated RMS outputs of each band filter. Options are no weighting or the use of one of the standard IEC A, B, C, or D curves.

Note

The RMS output calculation for each band pass filter is done using an exponential averaging method. The time constant of the averaging is different for each band, and is determined such that a constant number of periods (at the band center frequency) are being averaged. This means that the settling time of the RMS output amplitudes will be different for each band, and tend to be slower at the low end of the spectrum.