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I am using periodic triangular wave (Vp-p=3V, freq= 2kHz), as an input to my oscilloscope to do FFT. My question is that does the magnitude(y axis) of in FFT plot represent the peak to peak contribution or the RMS value of each sinusoids? Is there a difference between the y axis representation of FFT and simply a fourier transform? I am having hard time understanding the y-axis in both FFT and fourier transform representation plots. Help!

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Peak to peak, amplitude, and RMS are all related by constant factors. However, the scope display is usually shown in dBV or similar in terms of amplitude. Note that the scope also performs windowing before calculating the FFT. I have an Agilent MSO7104A that displays its FFT in dBVrms where 0 dBV is 1 Vrms, though this may not be an industry standard.

The FFT (fast fourier transform) is an algorithm that calculates the DFT (discrete fourier transform) which is the discrete version of the Fourier transform. The y-axis is fundamentally the same (complex phasor (amplitude and phase) for each frequency component) but the DFT works with discrete frequencies while the FT works with continuous frequencies. IOW, the DFT is a 'binned' version of the FT so you have a countable number of frequency bins instead of a continuous function.

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  • \$\begingroup\$ You are right. But is that dbV in RMS or peak to peak? \$\endgroup\$ – dr3patel Jan 22 '15 at 0:15
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    \$\begingroup\$ surely that would be scope specific? Tektronics scopes (sorry only ones I am really familiar with) provide it as RMS, you can choose linear or dB, but it is still an rms quantity \$\endgroup\$ – JonRB Jan 22 '15 at 0:42
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    \$\begingroup\$ Well, the FT itself gives you the amplitude (peak to peak over two) so the scope could rescale that to RMS. Generally when it is converted to dB it gets squared anyway (20 log) so really the distinction will be a constant offset. RMS could be the de facto standard, as Agilent scopes seem to display in dBVrms as well. \$\endgroup\$ – alex.forencich Jan 22 '15 at 1:03
  • \$\begingroup\$ Yes, I am just experimented what Barry said. I am now confident enough to say that it was Vrms (for agilent). \$\endgroup\$ – dr3patel Jan 22 '15 at 4:48
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To be sure, input a sine wave of known amplitude and check to see what the scope displays. For example, if the input is 2 volts peak-to-peak, the scope will indicate 2 volts if it is displaying peak-to-peak, 1 volt if it is displaying peak, and 0.707 volts if it is displaying RMS.

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  • \$\begingroup\$ That worked out. It is Vrms! \$\endgroup\$ – dr3patel Jan 22 '15 at 4:49

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