Register to reply

Phase distortion near nyquist frequency

by truva
Tags: distortion, frequency, nyquist, phase
Share this thread:
truva
#1
Nov8-11, 07:39 PM
P: 18
Hello all,

I obtained transfer function of a 8th order low-pass butterworth filter by bilinear transformation with frequency prewarping. When I plot the phase response of the filter for a given interval of frequency there are spreaded points near Nyquist frequency. (I used unwrap.m before plotting). What is the reason of this?

I also followed a different path: First defined an impulse (delta) function in the time domain and filtered it. And I used fft to obtain the transfer function. (I took into consideration the time shift effect)

The two methods gave the same phase response except at near the Nyquist frequency. Both of them included some spreaded points near Nyquist frequency. Can you tell me what is this?

I should add that the situation gets worst with the increasing order of the filter. And first method has lesser distorsion.
Phys.Org News Partner Engineering news on Phys.org
Identifying long-distance threats: New 3D technology could improve CCTV images
Intelligent system that predicts when alarms might be triggered could greatly improve the management of industrial plant
3D printed nose wins design award
truva
#2
Nov9-11, 06:41 PM
P: 18
When I use high-pass filter same thing happened at near zero frequency not at Nyquist frequency. Hence in both cases distortion is in stop band where the gain is about 1e-15. Therefore I can say that it is not a problem.

But I dont know the reason. May be it is beause of bilinear approximation or fft is making all the mess. I dont know but it is not important any more as I said.


Register to reply

Related Discussions
Continuous and Discrete Fourier Transform at the Nyquist frequency General Math 0
Maginutude and Phase Angle for Nyquist Plots Engineering, Comp Sci, & Technology Homework 1
DFT: What is the physical meaning of the symmetry about the nyquist frequency? General Math 0
FFT and Nyquist frequency - higher frequencyresolution with lesser samples Electrical Engineering 13
Nyquist (Polar) plots in circuits, phase question Advanced Physics Homework 1