title: How to write lowpass filter for sampled signal in Python?
body: <p>Sorry for question that may be trivial, but I am new in signal processing, and I didn't find python code with good explanation. So I will be glad to get explanation for dummy :)</p>
  
  <p>I have some signal that sampled each 1 nsec (1e-9 sec) and have, let say, 1e4 points.
  I need to filter high frequencies from this signal. Let say I need to filter frequencies higher than 10MHz.
  I want that for frequencies lower than cutoff frequency signal will be passed unchanged. It means gain of the filter will be 1 for frequencies lower than cutoff frequency.
  I would like to be able to specify filter order. I mean, fitsr order filter have 20 db/decade slope (power rolloff) after cutoff frequency, second order filter have 40 db/dec slope after cutoff frequency and so on.
  High perfomance of code is importent.</p>
  
  <p>Thank you very much for help.
  Alex.</p>