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Where to learn about "analog prototype filters"?

I've heard about them, but I'm unsure about what they really are and how they're constructed.

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"Analog prototype" filters are well-known analog filters that have specific desirable properties. They include (but aren't limited to):

These prototypes can be used to design digital filters that have approximately the same characteristics, for instance by using the bilinear transform.

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  • $\begingroup$ Why are they called analog? $\endgroup$ – mavavilj Aug 16 '18 at 15:53
  • $\begingroup$ Because they are physical objects that operate on analog signals. $\endgroup$ – Jason R Aug 16 '18 at 15:55
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    $\begingroup$ Note that for the Bessel filter, the bilinear transform will not result in a discrete-time filter that is optimal in any sense, because the frequency warping will affect the maximally flat group delay property. All other analog filters mentioned in this answer will retain their optimality when transformed by the bilinear transform. $\endgroup$ – Matt L. Aug 16 '18 at 16:34
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    $\begingroup$ @mavavilj they are “analog” in the sense that one can implement the filter in the continuous time domain (via design using Laplace transforms) using analog electronic components (resistors, inductors, capacitors), rather than discrete digital filters using the Z-transform. See Matt L.’s response regarding the bilinear transform as it’s a very important thing to note. $\endgroup$ – matthewjpollard Aug 17 '18 at 2:03
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    $\begingroup$ Also resist the temptation to design a digital filter by copying an analog prototype (unless there is specific reason to do so, such as modelling an analog system). In most cases direct digital filter design algorithms will result in the best filter implementation. Analog is limited by the realizable components (inductors, capacitors etc) while digital is limited by the math and precision available. This holds true for many aspects of signal processing design where a relationship to a known analog process can be instructive, but be cautious in simply copying an analog implementation. $\endgroup$ – Dan Boschen Aug 18 '18 at 1:47

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