Zoom Power Spectrum Express VI

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NI Sound and Vibration Measurement Suite 2009 Help

Edition Date: June 2009

Part Number: 372416C-01

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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.

Details

Dialog Box Options

Block Diagram Inputs

Block Diagram Outputs

Dialog Box Options

Parameter	Description
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. Input signal—Configures the measurement for a single channel or N channels. Changing this selection changes the data types of the input signals and computed measurements.
Configuration	Specifies the configuration parameters. Window—Specifies the window to apply to the input signal. Choose from one of the following window options: None Hanning (default) Hamming Blackman-Harris Exact Blackman Blackman Flat Top 4 Term B-Harris (Four Term Blackman-Harris) 7 Term B-Harris (Seven Term Blackman-Harris) Low Sidelobe Gaussian Scaling—Specifies the scaling parameters. Spectrum type—Specifies if the zoom power spectrum is in units of Magnitude or Power, where power equals magnitude squared. The default is Power. Magnitude scale—Specifies if zoom power is in linear units or in decibels. The default is decibels. Peak conversion—Specifies the peak scaling of the zoom power spectrum. You can select RMS, Peak, or Peak-to-Peak. The default is RMS.
Zoom Settings	Specifies the zoom setting parameters. Frequency Resolution Settings—Specifies the frequency resolution settings for the zoom power spectrum. You also can use the green, vertical cursors in the Spectra graph to set frequency range. Start frequency (Hz)—Specifies the lowest frequency of the zoom power spectrum, in hertz. You also can use the green, vertical cursors in the Spectra graph to set Start frequency (Hz) and Stop frequency (Hz). The default is 1k Hz. Stop frequency (Hz)—Specifies the highest frequency of the zoom power spectrum, in hertz. You also can use the green, vertical cursors in the Spectra graph to set Start frequency (Hz) and Stop frequency (Hz). The default is 2k Hz. Number of lines—Specifies the number of spectral lines to compute between start frequency and stop frequency. The default is 400 spectral lines. % Overlap—Specifies the percentage of data reused by consecutive FFT computations. You can use % Overlap to make the zoom more responsive when the values of Frequency range and Number of lines require a longer measurement time. The default is 0%. Frequency Resolution—Returns the frequency resolution parameters. Baseband resolution—Returns the frequency resolution of the baseband spectrum, in hertz. Zoom resolution—Returns the frequency resolution of the zoom spectrum, in hertz. Acquisition Time—Returns the acquisition time required to compute the first spectrum and subsequent spectra. First spectrum—Returns the time, in seconds, required to acquire data to compute the first zoom spectrum. The value depends on Start frequency, Stop frequency, and Number of lines. For the data acquisition time required for subsequent spectra, refer to New spectrum. New spectrum—Returns the time required, in seconds, to acquire data in order to get any spectrum after the first one. This value depends on Start frequency, Stop frequency, Number of lines, and % Overlap. Available—Returns TRUE every time a new zoom power spectrum computes.
Averaging	Contains the following options: Averaging Settings—Specifies the averaging parameters. Averaging mode—Specifies the averaging mode from the following options: No Averaging (Default) RMS Averaging Peak Hold Weighting mode—Specifies either Exponential or Linear weighting. Exponential averaging applies more weight to the most recent data, and linear averaging applies equal weighting to all the data. Select an Averaging mode to enable this option. Number of averages—Specifies the number of averages used by the selected Weighting mode when Averaging mode is RMS averaging. Auto-restart—[Weighting mode: Linear] Specifies if the averaging process automatically restarts once the step reaches the Number of averages value. Use Auto-restart to configure averaging to automatically restart when Averages completed equals the Number of averages. Status—Returns status information about the averaging process. Averages completed—Displays the number of averages completed. Averaging done—Indicates when the number of Averages completed equals or exceeds the Number of averages.

Block Diagram Inputs

Parameter	Description
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. start frequency—Specifies the lowest frequency of the zoom power spectrum, in hertz. The default is 1,000 Hz. stop frequency—Specifies the highest frequency of the zoom power spectrum, in hertz. The default is 2,000 Hz.
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.

Block Diagram Outputs

Parameter

Description

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.

Note You must wire spectrum info when performing extended measurements. Do not modify the spectrum info values. If you want to view the information, you can right-click the spectrum info wire and select Custom Probe»Spectrum Info from the shortcut menu.

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.

Zoom Power Spectrum Details

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.