[SI-LIST] Re: Rise time bandwidth relation
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 8 May 2007 13:20:37 -0400
> Can somebody help me in understanding the derivation of the following
> formula for digital signals:-
> Bandwidth = 0.35/Rise time.
Don't they teach that in college anymore?
I guess I shouldn't be surprised..... :-(
Someone on this list (I think it was Howie Johnson, but might have been
someone else) wrote a lengthy discussion about this formula and others like
it, on the SI-List a few years ago, maybe a couple of times. (Some form of
this question comes up every couple of years.) He derived the formulas for
the most common cases of theoretical interest, including single pole and
gaussian responses. Maybe you can find it if you search the archives.
> In the
> instance of an infinite pole, all pass filter a.k.a coax cable, I've read
> that BW ~ .31/Tr (10% to 90%). Can anyone clarify the origin of THAT
> number?
Certainly not!
Or was that meant as a trick question?
An all-pass response such as a lossless coax cable, would have infinite
bandwidth, so the formula above would be meaningless.
Or perhaps you meant lossy coax with some assumed low-pass response...?
Regards,
Andy
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