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2016
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Kirt
Blattenberger,
BSEE  KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...
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A filter's equivalent noise bandwidth (EqNBW) is the bandwidth that an ideal filter (infinite rejection in the stopband) of the same bandwidth would have. EqNBW is calculated by integrating the total available noise power under the response curve from 0 Hz to infinity Hz. In practice, integration only needs to be carried out to about the point of thermal noise. The steeper the filter skirts (higher order), the narrower the range of integration needed to get an acceptable approximation. Integration needs to be done in linear terms of power (mW, W, etc.) rather than in dB.
The values in the following table are for normalized lowpass filter functions with infinite Q and exact conformance to design equations. If you need a better estimation than what is presented here, then a sophisticated system simulator is necessary.
Butterworth (fco = 3 dB) 
Chebyshev (fco = ripple) 
Bessel (fco = 3 dB) 




Reference: Filter Design, by Steve Winder