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# FILTER design---BPF,BRF

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#### electronics_kumar

ideal filter characteristics, lpf, bpf, hpf

I GOT WHILLE REFERRING FILTER DESIGN THAT FILTERS SUCH AS BPF & BRF CAN BE BUILT BY CASCADING LPF & HPF....WITH STRIGENT REQUIREMENTS THAT FOR BRF---LPF FOLLWOED BY HPF WHEREAS BPF----HPF FOLLOWED BY LPF.
WHAT WILL HAPPEN IF WE INTERCHANGED LPF,HPF POSITON TO REALIZE THE BPF AND BRF IE is it possible to realize BPF ---->LPF follwed by HPF
BRF ----->HPF follwed by LPF

You can read "Design with Operational Amplifiers and Linear Integrated Circuit" by Sergio Franco.

brf filter

theortically speaking it will not affect anything on the system that you have there, but the only thing that could affect it is the cut off frequency of the two filters, once its equal

bpf lp hp

electronics_kumar,
Consider the case of a BRF consisting of ideal (Brickwall) LPF and HPF. If the HPF came first, all low frequencies below the high pass cutoff frequency would be rejected. There would be nothing for the LPF to pass. A similar argument holds for the BPF. If the LP filter came first, all high frequencies above the low pass cutoff frequency would be rejected. There would be nothing for the HPF to pass.
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In the real world, where brickwall filters are not attainable, it is possible (but not a good idea) to reverse the order. Consider the case of a BPF with 6 db/octave slopes. If the LPF comes first, the HPF would have to be a 2nd order filter. One order cancels the 6db slope of the the 2nd order provides the additional 6db/octave.
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This 2nd paragraph of this explanation applies to the case where the BPF, BRF is implemented by cascading LP,. HP filters.
Regards,
Kral

lfilter design bpf

I'm no filter expert, but I don't think the other responders here are either.
I can't see how it would be possible to make a band-reject filter by cascading HP and LP sections. They have to be in parallel.
The only reason I can see for the LP-HP order in a BPF is in the case of an active filter, where fast risetimes would be more likely to overload a HPF if it were first in line, while placing the LPF first would eliminate this possibility.
It really depends on the content of the signal to be filtered. If the signal has large low frequency content and small high frequency content, perhaps the HPF-LPF order would be preferable.

The answer is in this book: L.P. Huelsman "Active and Passive Analog Filter Design". In this book you will find many examples.

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