# $Q$ factor refers to what on a lowpass filter? (EE kind of definition)

For what I'm learning, the bandwidth (of a BPF, for example) refers to the $\Delta$ between $f_L$ and $f_H$ at (usually) $-3\textrm{ db}$ from the $f_0$ (from wiki). For a peaking filter instead, $f_L$ and $f_H$ are situated (usually) at the peak $\textrm{dB Gain}/2$.

Q Factor become $f_0 /\Delta$. It's clear till here.

• Now, if I take a lowpass filter, how is calculated $Q$ Factor?
• I have $f_0$, but how would I calculate the $\Delta$ of a lowpass?

There is no a "peak" as reference, so $-3\textrm{ dB}$ (or $\textrm{dB Gain/2}$) make no sense as references points for the "created" bandwidth.

• Can you please clarify if you are referring to low pass filters with resonance?. In that case, perhaps the points you are looking for are apparent from its frequency response (?) – A_A Nov 21 '16 at 11:37
• actually paizza, the definition of Q for the peaking filter is more of a cookbook thing than it is "usually". we had a discussion about this not too long ago. – robert bristow-johnson Nov 22 '16 at 17:06
• Not sure if its really only a "cookbook" thing :) From this : The threshold value is often defined relative to the maximum value, and is most commonly the 3dB-point, that is the point where the spectral density is half its maximum value (or the spectral amplitude, in V or V/Hz, is more than 70.7% of its maximum). It also seems that "your" dbGain/2 band edges (for the Peaking filter) refers to Half power point (which is again -3db to the peak). It looks like all is related. – markzzz Nov 22 '16 at 17:48
• Here (more or less) I'm asking if a Low Pass Filter's band edge are situated at -3db as for a Band Pass Filter (for the standard convention). I see this "-3db" only when talking about band pass (or notch)... – markzzz Nov 22 '16 at 17:49