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Design >> Analog Design >> Q of the low pass filter
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Message started by Analoger on Oct 2^nd, 2005, 10:41pm

Title: Q of the low pass filter
Post by Analoger on Oct 2^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!

Title: Re: Q of the low pass filter
Post by Analoger on Oct 4^th, 2005, 8:23pm

Any comments, pls?

Title: Re: Q of the low pass filter
Post by Anuranjan Jha on 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

Title: Re: Q of the low pass filter
Post by Analoger on 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.

Title: Re: Q of the low pass filter
Post by Paul on 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

Title: Re: Q of the low pass filter
Post by Analoger on 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.

Title: Re: Q of the low pass filter
Post by Paul on 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

Title: Re: Q of the low pass filter
Post by Juddy on 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!!!