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Design >> Analog Design >> How to measure the Q factor of a low pass filter in Cadence?
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Message started by josvlsi on Apr 3^rd, 2018, 12:02am

Title: How to measure the Q factor of a low pass filter in Cadence?
Post by josvlsi on Apr 3^rd, 2018, 12:02am

In cadence after plotting the AC gain and phase of a given filter, how can we determine the Q factor of the given filter??
I tried doing pz analysis but that displays a number of Q factors for different poles and zeroes. How can we determine the Q factor of any given filter in Cadence?

Title: Re: How to measure the Q factor of a low pass filter in Cadence?
Post by buddypoor on Apr 3^rd, 2018, 1:07am

I suppose you are referring to the Q factor Q=Qp of a second-order lowpass, correct?

Well - it`s very simple:
At the pole frequency wp the magnitude of the transfer function is A=Ao*Qp (Ao=Gain at DC).
The pole frequency wp can be identified very quickly: It is the frequency where the phase shift is exactly -90 deg.

Title: Re: How to measure the Q factor of a low pass filter in Cadence?
Post by josvlsi on Apr 3^rd, 2018, 2:18am

Yes, it is a second order low pass filter.
how did you get A=Ao*Qp (Ao=Gain at DC). ?

Title: Re: How to measure the Q factor of a low pass filter in Cadence?
Post by Jacki on Apr 10^th, 2018, 11:41pm

Hi buddypoor,

Is the pole frequency exactly at -90 deg or -45 deg?

Title: Re: How to measure the Q factor of a low pass filter in Cadence?
Post by josvlsi on Apr 11^th, 2018, 12:04am

The pole frequency can take any value depending on the filter design. I want to know how can we find the Q factor for any active second order filter simulated in Cadence