I need to design an antialiasing filter.
After reading a lot on them I found AN699 from Microchip by Bonie Baker which is clear enough except in one point:
When deciding on the transition band of the filter, given fS (sampling frequency) she states that fcut-off of the filter can be made much lower than fS/2.
To that, in page 5 she gives this specific examples:
Assuming a 5th order filter is used in this example:
fCUT-OFF = 0.18fS /2 for a Butterworth Filter
fCUT-OFF = 0.11fS /2 for a Bessel Filter
fCUT-OFF = 0.21fS /2 for a Chebyshev Filter with
0.5dB ripple in the pass band
fCUT-OFF = 0.26fS /2 for a Chebyshev Filter with
1dB ripple in the pass band
My questions:
Isn't contradictory that the filter has a cut-off frequency much lower than most of the frequencies of interest?
Accepted that the above is correct, how does she gets those coefficientes which apear as dependent of the filter's order? Up to now they seem drawn from a magic hat...
What I decide first? Order of the filter or cut-off frequency? Chicken or egg dilemma.
Sorry but this is my first time I face this subject.
Any help is appreciated.
After reading a lot on them I found AN699 from Microchip by Bonie Baker which is clear enough except in one point:
When deciding on the transition band of the filter, given fS (sampling frequency) she states that fcut-off of the filter can be made much lower than fS/2.
To that, in page 5 she gives this specific examples:
Assuming a 5th order filter is used in this example:
fCUT-OFF = 0.18fS /2 for a Butterworth Filter
fCUT-OFF = 0.11fS /2 for a Bessel Filter
fCUT-OFF = 0.21fS /2 for a Chebyshev Filter with
0.5dB ripple in the pass band
fCUT-OFF = 0.26fS /2 for a Chebyshev Filter with
1dB ripple in the pass band
My questions:
Isn't contradictory that the filter has a cut-off frequency much lower than most of the frequencies of interest?
Accepted that the above is correct, how does she gets those coefficientes which apear as dependent of the filter's order? Up to now they seem drawn from a magic hat...
What I decide first? Order of the filter or cut-off frequency? Chicken or egg dilemma.
Sorry but this is my first time I face this subject.
Any help is appreciated.