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Chapter 7: Lowpass & Highpass Filter Cha... > 7.13 More Complex Filters—Adding Zer...

7.13  More Complex Filters—Adding Zeros

All the filter types we have looked at so far can be made by plugging together second-order and first-order lowpass or highpass stages in cascade. This is true no matter how high the filter order. They are technically known as “all-pole filters” which basically means that they combine different sorts of roll-off, but once it is properly begun the response stays rolled-off; it does not come back up again. There are no deep notches in the amplitude response; once the roll-off is established it just keeps on going down. However, filters with notches in their stopband that plunge to the infinite depths (in theory, at least) have their uses, and their study makes up a large proportion of filter theory.

When a faster roll-off than Butterworth is required, without the passband amplitude ripples of the Chebyshev, one possibility is the Inverse Chebyshev, which adds a notch in the response just outside the passband. Notches have steep and accelerating slopes as you approach the actual notch frequency, so the roll-off is much accelerated.


  

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