Owning Palette: Frequency-Domain Measurements Express VIs
Requires: Sound and Vibration Measurement Suite
Computes the averaged zoom magnitude spectrum or zoom power spectrum. This measurement can return the spectrum in root-mean-square, peak, and peak-to-peak units. Data accumulates each time you pass new data. However, zoom analysis returns a new spectrum only when sufficient data accumulates.
With the zoom power spectrum, you can specify the zoom settings used in the zoom analysis, such as the resolution settings to control the frequency. The number of lines used determines the number of lines computed in the zoom power spectrum.
|Dialog Box Options|
|Block Diagram Inputs|
|Block Diagram Outputs|
|View||Specifies the display that appears in the Signals graph.|
|Spectrum||Displays the baseband power spectrum and the zoom power spectrum if enough data has accumulated. Use the Zoom button to zoom in and out of the display. If you select Frequency spectra from the View pull-down menu, you can use the Cursor button to move the cursors. You can use these two cursors on the Frequency spectra graph to set the Start frequency and the Stop frequency.|
|Autoscale||Automatically adjusts the scales of the graph to display the data.|
|Input||Specifies whether to operate on a single channel or on N channels. This option is available only with the NI Sound and Vibration Assistant.|
|Configuration||Specifies the configuration parameters.|
|Zoom Settings||Specifies the zoom setting parameters.|
|Averaging||Contains the following options:|
|input signal||Specifies the input signals.|
|error in (no error)||Describes error conditions that occur before this node runs.|
|frequency range||Specifies the frequency range of the zoom power spectrum.|
|number of averages||Specifies the number of averages used by the selected Weighting mode when Averaging mode is RMS averaging.|
|number of lines||Specifies the number of spectral lines to compute between start frequency and stop frequency. The default is 400.|
|restart averaging||Specifies whether to restart the selected averaging process. The default is FALSE. When you call this Express VI for the first time, the averaging process restarts automatically.|
|zoom spectrum||Returns the zoom power spectrum.|
|spectrum info||Returns spectrum information. This information is needed by the Peak Search Express VI and the Power in Band Express VI. |
|unit label||Returns the unit label for the input signal.|
|averages completed||Returns the number of averages completed by the Express VI.|
|averaging done||Returns TRUE when the number of averages completed equals or exceeds the number of averages specified in averaging parameters. Otherwise, averaging done returns FALSE.|
|error out||Contains error information. This output provides standard error out functionality.|
|new spectrum||If TRUE, new spectrum indicates that zoom spectrum contains a new measurement and is not empty. When you wire the output value to the selector terminal of a Case structure, you can perform subsequent measurements or display the results in the TRUE case of the Case structure.|
Real-time zoom is also known as destructive zoom because the input time waveform data is processed through stages of filtering and decimation before reaching accumulation. When a sufficient number of processed samples have been accumulated, you can use the zoom power spectrum to perform the zoom transform to measure the spectrum within the specified frequency range. When an application requires fine frequency resolution over a narrow span of the baseband frequency range, the zoom power spectrum can be used to reduce memory and processing requirements because the transform is performed on a decimated set of data.
You can use the zoom power spectrum to perform online averaged spectral analyses on a continuous series of blocks, as might be acquired in a continuous acquisition. The data must be continuous as measured by the timestamp and the sample rate. The block size does not have to be constant because it effectively decouples the input block duration from the spectral frequency resolution. When sufficient data has been accumulated, the zoom power spectrum will return an updated averaged spectrum and the new spectrum available output will be TRUE. It is common to put the zoom power spectrum inside an acquisition or processing loop and continuously input time signals until the spectral result is done. It will continuously output updated averaged spectra as they are computed.
You can use the zoom power spectrum to perform offline averaged spectral analyses on a single, long-duration waveform, as might be acquired in a Finite acquisition or File Read operation. Provided that sufficient data is available, the zoom power spectrum may be used to decouple the frequency resolution from the duration of the signal. Instead, you can measure an averaged spectrum at a coarser frequency resolution than the baseband resolution for the long-duration signal, as expressed by baseband df = 1 / (N*dt). The zoom power spectrum will output the last averaged spectrum if one is available.