Consider following example:

import numpy as np
import scipy as scp
import matplotlib.pyplot as plt

#make signal
deltat = 1e-4
t = np.linspace(0,300000,300001) * deltat
y = np.exp(-t/10)*np.sin(np.pi * (t/4)**2)

#plot signal
plt.figure(1)
plt.clf()
plt.plot(t, y)
plt.show()

#calculate spectrogram
f, t, Sxx = scp.signal.spectrogram(y, 1/deltat)

#plot spectrogram
plt.figure(2)
plt.clf()
plt.pcolormesh(t, f, Sxx)
plt.ylim([0, 100])
plt.show()

A sine with increasing frequency and decreasing amplitude is created and its spectrogram is plotted. Note that the sine is oversampled (it represents a measurement at fixed time step deltat).

The spectrogram should be an increasing line (increasing frequency) of decreasing color (decreasing amplitude). How can I obtain the correct spectrogram?

• Trying to include the nperseg and noverlap keywords in the spectrogram function does only give me other wrong spectrograms.
• Should I use an alternative Python function instead of spectrogram? Update: I have also tried using librosa.stft without success.
• Is the problem related to the oversampling?