buddypoor wrote on Dec 2nd, 2010, 12:21pm:Carl, I don't completely agree with you; I don`t like the term "strictly" in your answer. A definition is an arbitrary common agreement - nothing else.
And in some cases you are, of course, allowed to specify another "corner frequency", for example only 1 or 2 dB down. But in any case, it should be mentioned clearly as such.
Hi Buddypoor,
I guess I agree and disagree with you at the same time. You are quite right that people are free to specify different specifications (for example like you said a 2 dB corner frequency). However, there has to be some agreement to standard specifications or else there will be endless confusion. I think if someone has a circuit and says to you "fc of this circuit is 100 MHz" the person is saying "The frequency at which the output is 3 dB below it's midband value is 100 MHz" and only that. If the person actually means 2 dB or 10 dB down, it is the responsibility of that person to explicitly say "10 dB corner" or something like that. "Fc" without a qualification means 3 dB corner, and only that.
An example where there is a lot of confusion in practice is with Noise Figure. It *is* strictly defined at a 50 Ohm reference impedance but a lot of times people get confused about this, particularly when their gain stages drive high impedances.
So, I spent a lot of time saying: Yes, I agree specifications are arbitrary. However, once we agree they have to be absolute or else they lose a lot of their usefulness.
Perhaps we can agree on this much? :)
Carl