Quality factor of a low pass filter

[MohEllayali]

Hello everybody

I read few days ago this sentence 'Q-factor of a low-pass filter'
is that correct ??? I thought there was a quality factor only for bandpass filter
Q = f0/BW
so what could represent the Q-factor of a LPF , the out of steepness ???

thanks

 

[keith1200rs]

See https://en.wikipedia.org/wiki/Q_factor the section on Q and damping factors which can apply to filters.

Keith.

 

[MohEllayali]

Thank you for the reply Keith
Actually I've already read the wiki article
for a pass band filter it's obvious .
for a 2nd order low pass filter it's okay also if we say that the quality factor of the filter is that of the pair of poles
but when the order is 3 or bigger, I can't see what is the Q-factor of a low pass filter.

 

[keith1200rs]

It's not something I have used other than with bandpass filters. The Electronics Filter Design Handbook (Williams/Taylor) doesn't mention Q except for bandpass.

Keith.

Added after 2 minutes:

Nuhertz Filter Solutions help file says:

Q is only defined for second and third order stages. For second order stages, Q is defined as Wo divided by the sum of the real part of the pole locations. For third order stages, the two complex poles are used to calculate and display Q. If all poles are real, the two smaller poles are used. Q provides a measure of sharpness to the stage frequency response. High Q implies a very narrow sharp frequency response. Low Q provides a wide shallow frequency response. The minimum value for complex poles is 0.5. Q is dimensionless.

Stages are sequenced such that low pass and low Q stages are physically position prior to high Q and hi pass stages. Low pass takes precedence over low Q. Stages with equal Q are sequence by increasing Wo. This minimizes the possibility of op amp slew rate limiting, and internal signal ringing. Stages may be physically sequenced by the user by clicking on any op amp of any stage and selecting the stage number of the desired stage to switch places with.

Keith.

 

[MohEllayali]

Okayyyyyyy, thank you Keith,