# I want to plot PSD for my input data(meters)

Asked by suresh on 29 May 2012
Latest activity Commented on by Elige Grant on 29 May 2012

I want to plot PSD for my input data (meters) if the input in volts the the PSD in dB/Hz what if the input in meters then what are the units of PSD please some body help me Ts=30; Fs =1/Ts; y=MP1; %input data(meters) h = spectrum.welch; Hpsd=psd(h,y,'Fs',Fs); what are units of y axis after the plot

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Answer by Elige Grant on 29 May 2012

If you are measuring displacement in meters in the time domain, then your frequency domain amplitudes (after performing the FFT) should be in units of meters / Hz. Squaring those amplitudes should give you meters-squared / Hertz-squared. However... since Matlab generates their power spectra by normalizing their FFT result by the number of samples (N) instead of the sampling frequency (Fs), their output might have PSD units associated with meters-squared / samples-squared (which is completely meaningless to me).

suresh on 29 May 2012

sir, In h=spectrum.welch ...how many samples it will take to calculate the PSD.... my input data has 650 samples..

Elige Grant on 29 May 2012

It will take all of them... or you can re-parameterize your "h = spectrum.welch" to operate on a sub-set (or window) of your data with pre-defined overlap amounts. If you want the PSD of the full data, then you might have to do Hpsd = psd(h,y,'NFFT',650,'Fs',Fs) since in my test (running without 'NFFT',650) it performs a 256-point FFT. Check out the documentation for more specific examples of setting up/paramertizing these functions.

Answer by Wayne King on 29 May 2012

Is your sampling interval in meters, or are you really saying that your measured variable is in meters?

I'm assuming you just mean that your data is sampled spatially. What is actually being measured?

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suresh on 29 May 2012

sir, its a white noise (m) which is a input signal...i want to find PSD of input to known the Maximum power in the input signal
the thing is if input is in volt the units of PSD is dB/Hz what if the input signal unit are in meters then PSD units will change or not....