Highest frequency with 0 dB attenuation point

[atferrari]

Just for reference, let us say we are talking of a Butterworth low pass filter.

What is the real usefulness of knowing the cutoff frequency value (-3 dB attenuation) for me, when I am interested in designing a filter that would pass UP TO xxx Hz with no attenuation, that is 0 dB?

For this particular case, why cutoff frequency is not expressed as the highest frequency point where attenuation is still 0 dB for that filter?

Other way of asking more or less the same: how can I know for the above filter what is the highest frequency that would pass without being attenuated?

Since we are here, could anyone, after answering that question, extend the concept to all filters?

 

[JimB]

Many filters, particularly multistage LC filters do not have a completely flat passband but have some ripple say +/- 1dB.

So, attempting to define the passband as the completely flat part of the response is a bit of a waste of time.
Hence, the use of the -3dB points to define filter bandwidth.

JimB

 

[audioguru]

-3dB (half the power) has been selected as the cutoff frequency of a filter because it is a small but noticeable drop in response. Few people can hear a 1dB drop or see a 1dB drop in the response of a video amplifier.
The number of the "order" of a filter determines how fast its response drops. A single-order filter drops its output a little at maybe 1/10th or 10 times its cutoff frequency but a sixth-order filter might begin dropping at half or double the cutoff frequency.

 

[crutschow]

In the real world you need to define some tolerance of 0dB, whether it's 0.1dB or 0.01dB, or whatever, for your rolloff point. When you have determined that then you can design a filter for the required bandwidth. Convention is to define a filter at it's -3dB point, but if you want to define a different point then you just need to calculate that from the filter equations. But be aware that a small dB value for the rolloff point will mean that there may still be a lot of signal that is passed through between that frequency and the steep rolloff point of the filter.

 

[atferrari]

I am conscious of the rest of the concepts involved but the main concern in this case is the highest frequency not attenuated by the filter in question.