I have one doubt on choosing filter Bandwidth. My input signal frequency is 40 MHz for an active RC filter. I don't know how to choose value of the -3dB BW. Is there any relation between the input signal freq versus BW?
It depends squarely on Nth order rolloff for -3dB, ripple skirt shape etc.
But for 1st order approximation, 10ns rise time = 30MHz
1ns = 300MHz etc.
so tR = 3/10f when defined as 10~90% , not 0~ (1-1/e)=63%
Thus 40MHz= 7.5ns
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FWIW, learn to specify RF filters as bandpass ripple f and bandstop rejection point.
Eg flat to 40MHz +/- 0.x dB and reject y dB at f2, then add phase error and group delay specs as needed
Beside knowing what you need to reject, you have to know what you need to pass.
For example, passing a bunch of narrow band RF channels below 40 MHz is a different filter then passing a video signal that is critical to group delay.
An active LPF filter with corner at 40 MHz can be very tough to keep stable.