FFT and Tone Measurements

Question

Does the Express VI, Spectral Measurements and the Extract Multiple Tone Information VI return the same data?  The express VI displays the information in dB and the Extract Multiple Tone Information VI returns the amplitude in volts.  The issue is that the amplitude don't seem to match.  I am using 20log(volts) to convert to dB.  The chart shows about -14 dB and the Extract Multiple Tone Information VI returns -27 dB (volts convert to dB).
Any help is appreciated,
Matthew Fitzsimons
Certified LabVIEW Architect
LabVIEW 6.1 ... 2013, LVOOP, GOOP, TestStand, DAQ, and Vison

Answer 1

You are calculating a power spectal density in the spectral measurements Express VI.  If you configure it to calculate the spectral measurement magnitude (peak) the values will match.  The difference from the help file is:
Magnitude (peak)—Measures the spectrum and displays the
results in terms of peak amplitude. You typically use this measurement with more
advanced measurements that require magnitude and phase information. The
magnitude of the spectrum is measured in peak values. For example, a sine tone
of amplitude A yields a magnitude spectral value of A at the sine tone
frequency. You can unwrap the phase spectrum or convert it from radians to
degrees by setting Phase to Unwrap phase or
Convert to degree, respectively. If you place a checkmark in
the Averaging checkbox, the phase of the spectrum is zero for
averaging.
Power spectral density—Measures the spectrum and displays
the results in terms of power spectral density (PSD). Power spectral
density is a scaled version of Power spectrum, where
the power present within each spectral bin is normalized by the frequency bin
width. You typically use this measurement to examine the noise floor of a signal
or the power in a specific frequency range. Normalizing the power spectrum by
the bin width makes this measurement independent of the signal duration, or
number of samples.So, they are the same if they are the same.
Hope that this helps,
Bob Young
Bob Young - Test Engineer - Lapsed Certified LabVIEW Developer
DISTek Integration, Inc. - NI Alliance Member
mailto:[email protected]