Designer’s Guide Community

Analoger Junior Member Offline Posts: 15	Q of the low pass filter Oct 02^nd, 2005, 10:41pm Hi all, I was a bit confused. I understand that quality factor "Q" is an important spec of bandpass filter. But what does quality factor mean for low pass filter? Why do we need to care about it? Thank you!
Back to top	IP Logged

Analoger Junior Member Offline Posts: 15	Re: Q of the low pass filter Reply #1 - Oct 4^th, 2005, 8:23pm Any comments, pls?
Back to top	IP Logged

Anuranjan Jha Guest	Re: Q of the low pass filter Reply #2 - Oct 10^th, 2005, 11:19pm For a low pass filter, Q does not make sense unless you specify the frequency. You may say Q is a function of frequency because the loss in the circuit depends on it. Anuranjan
Back to top	IP Logged

Analoger Junior Member Offline Posts: 15	Re: Q of the low pass filter Reply #3 - Oct 15^th, 2005, 9:26pm Anuranjan , no offense, but i don't think you really understand the question and the definition of quality factor - which has different meanings under different circumstance.
Back to top	IP Logged

Paul

Community Fellow

Offline

Posts: 351
Switzerland

Re: Q of the low pass filter
Reply #4 - Oct 17^th, 2005, 12:12am

Hi,

I'm not sure who misunderstands the signification, in any case your question is ambiguous. I guess you are questioning the pole quality factor, not the filter quality factor. The pole quality factor defines who much damping your transient response shows, respectively how much peaking your frequency response exhibits. It essentially depends on the ration between the real and the imaginary parts of the poles. For a two-pole filter, you can find the math in any convenient text book. The following document discusses both quality factors:
http://www-inst.eecs.berkeley.edu/~ee247/fa04/lectures/L2_f04.pdf

Paul

Back to top

IP Logged

Analoger Junior Member Offline Posts: 15	Re: Q of the low pass filter Reply #5 - Oct 17^th, 2005, 10:31am Hi Paul, I understand the pole quality factor concept. The imaginary part of the pole determines the oscillation frequency, and the real part determines the damping. I remembered I saw it somewere that "Q" of a low-pass filter is given as a spec. That's why I'm asking. Thank you.
Back to top	IP Logged

Paul Community Fellow Offline Posts: 351 Switzerland	Re: Q of the low pass filter Reply #6 - Oct 19^th, 2005, 12:32am Quote: I remembered I saw it somewere that "Q" of a low-pass filter is given as a spec. But that certainly is the pole quality factor and not the filter quality factor. Best Paul
Back to top	IP Logged

Juddy New Member Offline Posts: 2	Re: Q of the low pass filter Reply #7 - Oct 23^rd, 2005, 6:55pm By the way, I have also a question on Q of the LPF. when we design a cascaded (two-stage) filter with each stage two poles, how to design the Q- of each stage, should the Q value of two-stages increase of decrease, thanks in advance!!!
Back to top	IP Logged