title: Why does scipy.signal.welch suppress zero frequency?
body: <p>I try to use whelch method and I found that zero frequency is abnormal</p>
  
  <pre><code>import numpy as np
  import scipy.signal as signal
  import matplotlib.pyplot as plt
  
  n = 100000
  s = np.ones(n)
  f, psd = signal.welch(s, return_onesided=False)
  plt.plot(f, psd)
  plt.show()
  </code></pre>
  
  <p>So result PSD is zero. 
  Why does scipy.signal.welch suppress zero frequency? </p>