Need help to determine Q factor

[richloo]

Hi there, i have designed a Butterworth low pass filter, but i don't know how to measure Q factor from simulation to comfirm my design. Can u guyz help me out?
Thanks~!!!

 

[Vamsi Mocherla]

Usually we measure the Q factors only for Band pass and Band Reject filters. But you can get a measure of Q from the transient response and solving for the settling time.

Added after 4 minutes:

Hey you can actually, get the real pole locations from your simulation results using the transfer function analysis(Of course, the poles of the opamp will cause some change). Then you can get the TF and then get Q out of it. This is just a thought, though, I hever never done this myself

 

[richloo]

Thanks Vamsi, it sounds interesting, can u tell me how to setup the circuitry for transfer function (xf) analysis using spectre?

 

[Vamsi Mocherla]

Well, you run an ac analysis like you normally do. Then you should add the transfer function analysis to the deck.You should define the input voltage source where you will start measuring and the output node where you will stop measuring. Then the transfer function analysis should give you all the locations of poles and zeroes in between those two points.

You can look into SPECTRE manual for statements for the xf analysis.