SVFA Compensate Phase for Reference Frequencies VI

SVFA Compensate Phase for Reference Frequencies VI

Owning Palette: Embedded Monitoring VIs

Requires: Sound and Vibration Toolkit

Use this VI to enable vector averaging of complex spectra even when the frequencies of interest do not have constant phase incidence in the measurement window (analysis block).

To use this VI:

1. Identify reference frequency components.
2. Construct compensation vector to adjust phase of reference frequency components to zero.
3. Phase compensate complex spectrum by multiplying compensation vector with each input complex spectrum. Phase compensation makes all reference components coherent with the analysis block so that vector average preserves reference components amplitudes. See details for usage.

Note  No instances of SVFA Compensate Phase for Reference Frequencies maintain state internally. To process multiple data sets, call these Vis in a loop.

Details  

SVFA Compensate Phase for Ref Freq and Harm (Complex) (1 Ch)

This instance uses the identified phase of the fundamental reference frequency [Hz] as a measure of the relative delay between the signal and the analysis block. The phase compensation vector is constructed to eliminate the linear phase due to this delay.

	complex spectrum specifies the complex spectrum response. f0 specifies the start frequency, in hertz, of the spectrum. df specifies the frequency resolution, in hertz, of the spectrum. complex spectrum specifies the complex form of the computed spectrum. spectrum info specifies the properties of the previously computed spectrum. 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 unbundle and right-click the spectrum info wire and select Custom Probe»Spectrum Info from the shortcut menu.
	reference frequency [Hz] specifies the frequency in Hz at which the spectral density of the ideal inverse-f noise waveform is equal to noise density.
	advanced span [Hz] specifies a frequency span centered at the identified fundamental frequency in which noise and spurious components are ignored for the measurement. Specify a value of 0 if the frequency is known and time invariant. Specify a value of –1 to permit the measurement to automatically compute the minimum frequency span suitable for the time-domain window applied to the signal. The default is –0 Hz.
	error in describes error conditions that occur before this node runs. This input provides standard error in functionality.
	phase compensated complex spectrum returns the phase compensated complex spectrum.
	error out contains error information. This output provides standard error out functionality.

SVFA Compensate Phase for Ref Frequencies (Complex) (1 Ch).vi

This instance uses the identified phase of each reference frequency [Hz] as a local measurement of relative delay of that component with respect to the analysis block. The phase compensation vector is constructed to adjust the reference component phases to zero.

	complex spectrum specifies the complex spectrum response. f0 specifies the start frequency, in hertz, of the spectrum. df specifies the frequency resolution, in hertz, of the spectrum. complex spectrum specifies the complex form of the computed spectrum. spectrum info specifies the properties of the previously computed spectrum. 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 unbundle and right-click the spectrum info wire and select Custom Probe»Spectrum Info from the shortcut menu.
	reference frequencies [Hz] specifies the frequencies in Hz at which the spectral density of the ideal inverse-f noise waveform is equal to noise density.
	advanced span [Hz] specifies a frequency span centered at the identified fundamental frequency in which noise and spurious components are ignored for the measurement. Specify a value of 0 if the frequency is known and time invariant. Specify a value of –1 to permit the measurement to automatically compute the minimum frequency span suitable for the time-domain window applied to the signal. The default is –0 Hz.
	error in describes error conditions that occur before this node runs. This input provides standard error in functionality.
	phase compensated complex spectrum returns the phase compensated complex spectrum.
	error out contains error information. This output provides standard error out functionality.

SVFA Compensate Phase for Specific Orders (Complex) (1 Ch).vi

This instance converts the reference speed, in RPM, to a reference frequency, in Hertz. The reference frequency is used to convert input orders to a list of reference frequencies. This VI then calls SVFA Compensate Phase for Reference Frequencies VI.

	complex spectrum specifies the complex spectrum response. f0 specifies the start frequency, in hertz, of the spectrum. df specifies the frequency resolution, in hertz, of the spectrum. complex spectrum specifies the complex form of the computed spectrum. spectrum info specifies the properties of the previously computed spectrum. 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 unbundle and right-click the spectrum info wire and select Custom Probe»Spectrum Info from the shortcut menu.
	speed [RPM] specifies the rotational speed, in revolutions per minute, for converting the input frequency power spectrum.
	orders to track specifies the orders for which you want to compute the magnitude and phase.
	error in describes error conditions that occur before this node runs. This input provides standard error in functionality.
	phase compensated complex spectrum returns the phase compensated complex spectrum.
	error out contains error information. This output provides standard error out functionality.

SVFA Compensate Phase for Reference Frequencies Details

This VI is used with vector averaging to improve signal-to-noise ratio for specified frequencies of interest. The following VI generates a fundamental tone at 1999.9 Hz and adds broadband, incoherent noise. A complex FFT is computed (averaging = None). SVPO Average FFT Spectrum (Complex) is used to implement averaging of the complex frequency spectrum.

Running this VI produces spectra shown in the following graphs:

Vector averaging of the frequency spectrum improves signal-to-noise ratio, but must also be used with care because it effectively attenuates all signal components that are incoherent with the analysis block. Broadband energy, non-integer multiples of the fundamental frequency, and all signals with incoherent (random) phase incidence to the analysis block are attenuated by vector averaging of frequency spectra. Graph 1 shows that RMS averaging reduces the variance in the noise when compared to the spectrum of any one analysis block. Graph 2 shows that vector averaging attenuates all signal components incoherent with the analysis block, and it can be seen that frequency-referenced vector averaging preserves expected amplitude of the reference frequency component while lowering the noise floor. For demonstration purposes, test signal frequency was selected so that 100 vector averages would completely attenuate the 1 Vpeak (0.7071 Vrms) signal. Refer to RMS versus Vector Averaging for more information on averaging techniques.