How can I convert the units of the y-axis into miliseconds^2/Hz in power spectral density analysis of an ECG signal?

This is the code that plots the PSD in dB/Hz on the y-axis

nfft = 2^nextpow2(length(ecg));

Pxx = abs(fft(ecg,nfft)).^2/length(ecg)/fs;

Hpsd= dspdata.psd(Pxx(1:length(Pxx)/2),'fs',fs);

p = plot(Hpsd);

Answer 1

[Wayne King]

The y-axis on a PSD estimate will not be in units of the time squared, it will be in amplitude squared per Hz, so for an ECG signal I assume that will be in millivolts, not milliseconds. The recorded values of the waveform are not in time.

Since you are using dspdata.psd, why not just use spectrum.periodogram?

    psdest = psd(spectrum.periodogram,ecg,'NFFT',length(ecg),'Fs',fs);
    plot(psdest)

If you want to plot without using dB (that is preferred by the way)

   plot(psdest.Frequencies,psdest.Data)

[Aakash Mahawar]

u r wrong wayne king.....when psd estimate of an ECG signal is done....then y axis wud be ms^2/Hz.since it belongs to psd of RR time series.