# Computing Power Spectral Density from FFT vs Welch's method vs Periodogram [duplicate]

I'm looking into different ways to get the Power Spectral Density (PSD) (while toying around with python) of a discrete signal/time series, and I'm struggling to understand why I get (very) different outcomes using different approaches. In particular, when computing the PSD from the FFT of a signal - using the squared absolute values - the resulting scale is several orders of magnitude greater than what I get from alternatives like Welch's method or using a Periodogram:

import numpy as np
import matplotlib.pyplot as plt
from scipy.signal import welch, periodogram

# input
t = np.arange(5999.) / 300
dt = t[1] - t[0]
y = np.sin(np.pi * t) + 2 * np.sin(np.pi * 2 * t)
N = len(y)
fs = 1.0 / dt

# FFT
n = 2**(N - 1).bit_length()
Y = np.fft.fft(y, n)
Ypos = Y[0:int(n / 2 - 1)]
ff = (fs / n) * np.arange(0, n / 2 - 1, 1)
Yre = np.real(Ypos * np.conj(Ypos) / n)

# PSD
wf, wPSD = welch(y, fs, nperseg=N)
pf, pPSD = periodogram(y, fs, scaling='density')
fPSD = (np.abs(Yre) ** 2) / (N / fs)

# plots
figy, axy = plt.subplots()
axy.set_title('y(t)')
axy.set_xlabel('time(s)')
axy.plot(t, y)
figS, axS = plt.subplots(3, 1, constrained_layout=True)
figS.suptitle('$$S_{yy}(f)$$')
axS[0].set_title('welch')
axS[1].set_title('periodogram')
axS[2].set_title('FFT')
axS[1].sharex(axS[0])
axS[2].sharex(axS[1])
axS[0].plot(wf, wPSD)
axS[1].plot(pf, pPSD)
axS[2].plot(ff, fPSD)
axS[0].set_xlim(0, 3)
axS[1].set_xlim(0, 3)
axS[2].set_xlim(0, 3)
axS[2].set_xlabel('f (Hz))')
plt.show()


which plots the following spectra:

I understand that Welch's method and the Periodogram are simply estimates of the signal's PSD, and some discrepancy is to be expected. But how exactly should I scale the values when computing the PSD directly from the FFT output, in order to achieve somewhat comparable results?

On the other hand, one would expect any power calculation to require the input of any time information, contrary to the FFT which can be scaled to fit different sampling rates.

• Yeah, depending on the implementation and method, getting the scaling right can be tricky. This is a recurrent question. Please see this answer, as well as this one and if those don't help, let me know and I'll help further.
– Jdip
Commented May 4, 2023 at 22:08
• I don't know if it's a duplicate but perhaps it's too broad, OP should clarify after checking what users referenced, which appear sufficiently related. Commented May 5, 2023 at 19:00