-3db and filter's

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large_ghostman

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I have been doing some reading on filters, mainly low pass, high pass and band pass. No real reason for this, just learning the basic's. Alot of what i have read, indicates that the filter is characterised at the -3db point.
So my question is, why is the value 3 db chosen? is it just a random figure that was chosen (a bit like 10 - 90% for rise times on a scope) or is there a technical reason that it has to be 3db?
 
Thank you Jim, that is exactly what i was after! So in a way it is exactly like why a rise time measurement on a scope is taken between 10% and 20%, that way you have a definite cut off point, instead of a vague end point due to say noise .
I like to have the reason's and stuff written in my book. I have been playing with filters a little, net step is to try and get the spectrum analyzer working gain! No idea why it isnt working, but since we moved it is playing up. Would be nice to see watch the filter's in the frequency domain as well as the time domain
 
JimB said:
10 and 90%
A typo I presume.

JimB
Click to expand...
LOL I even proof read it! Yes dont worry it was a typo, no idea how you make that kind of typo, isnt like they are near each other on the keyboard I got it correct in the first post, then had brain to keyboard malfunction :d
 
f=1/(2*PI*R*C) is the (-3db point or 1/Sqrt(2) * the input)

The error is fixed above as pointed out by Mr. Al.
 
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Thanks KISS another for the book , also found a good video that has helped also, as I learn these formula i am going to put them in excel for easy working, but for now it's a calculator job, and try and get some fixed in my head
 
In audio filters -3db point is also known as the "half power" point.
 
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