[Scipy-tickets] [SciPy] #1812: Erratic phase plot for high order filters

SciPy Trac scipy-tickets@scipy....
Sat Jan 12 20:51:47 CST 2013

#1812: Erratic phase plot for high order filters
 Reporter:  kitmonisit  |       Owner:  somebody   
     Type:  defect      |      Status:  new        
 Priority:  normal      |   Milestone:  Unscheduled
Component:  Other       |     Version:  0.11.0     
 Keywords:              |  
 PROBLEM: The phase plot computed from scipy.signal.lit.bode function shows
 abrupt changes between -180° and 180°.

 I'm using the scipy.signal module to design analog filters. These are the
 steps I used to do it (it's also commented in the code):

 1. Set up specifications
 2. Convert Hz to rad/s (w is "omega")
 3. Normalize w from 0 to 1
 4. Calculate required filter order and natural frequency
 5. Calculate zeros, poles, and gain (scipy.signal.butter)
 6. Create LTI system (scipy.signal.lti)
 7. Compute Bode plots (scipy.signal.lti.bode)
 8. Convert w back to f and scale appropriately
 9. Plot stuff to "plot.pdf"

 Attached files are butterworth.py and plot.png

Ticket URL: <http://projects.scipy.org/scipy/ticket/1812>
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