# butterworth digital low pass filter

Discussion in 'General Electronics Chat' started by vead, Jan 13, 2014.

Nov 24, 2011
712
11
butterworth digital low pass filter has following specification

passband-0-500 hz
stopband-2-4 khz
passband ripple-3 db
stopband attenuation-20 db
sampling frequency-8 khz

determine the passband and stopband edge frequency

please tell me the hint or formula to calculate passband and stopband frequency

2. ### Veracohr Distinguished Member

Jan 3, 2011
676
103
Surely whatever class or book this question came from also gave you the tools to answer it. Else what is the point of the class?

However, it doesn't make sense. A Butterworth filter shouldn't have passband ripple.

If you can assume it's a Butterworth filter (no passband ripple) or that it's a Chebyshev filter with 3dB passband ripple, then the beginning of your answer is right there. You have the minimum frequency your passband must extend to, you have the maximum frequency your stopband must start at, and you have the attenuation that must be seen at the start of the stopband. The class or book or whatever should have explained how to take those numbers and determine the filter order needed and cutoff calculation.

3. ### Fritz Kreisl New Member

Jan 13, 2014
1
0
butter designs lowpass, bandpass, highpass, and bandstop digital and analog Butterworth filters. Butterworth filters are characterized by a magnitude response that is maximally flat in the passband and monotonic overall.
<snip>

Last edited by a moderator: Jan 13, 2014

Nov 24, 2011
712
11
this is unsolved question I don't have anything to solve it i have googled and spend lot of time to find out but I didn't found anything

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,626
733
Ok. You have a transfer function. You plug a frequency into the transfer function, do the math, you come up with a number between 0 and 1.

When you plug a cutoff frequency of the filter into the transfer function, the result is 0.707, so 0.707=transfer function. Now solve it for the frequency.

6. ### LvW Well-Known Member

Jun 13, 2013
812
108
Some days ago you have opened another thread dealing with the same subject :

In my answer (post#2) I told you that there is no simple formula for calculating such a filter. First, you have to choose one of different design strategies available.

Nov 24, 2011
712
11
that was different post but topic is same I thought may be someone can help in this way but now I think no one can help me I have to do myself

8. ### LvW Well-Known Member

Jun 13, 2013
812
108
Rather, I think it is not possible to help you based on some rough information in this forum. The task is rather complex and requires more work than can be done by simply ask questions and give answers.

9. ### atferrari AAC Fanatic!

Jan 6, 2004
3,253
1,269
Not a bad idea, I think.