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Design >> Analog Design >> oscillaotr quality factor
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Message started by raja.cedt on Oct 22^nd, 2008, 9:51pm

Title: oscillaotr quality factor
Post by raja.cedt on Oct 22^nd, 2008, 9:51pm

hi , i have question regarding oscillator
1.generally Quality factor indicates slope of the phase response. ie Q=w*0.5*(dΦ/dw).can any body please explain intuitively this proportionality relation.more over i derived this relation for 2nd order.so does it applicable for other orders also,if so how to explain.
Thank you

Title: Re: oscillaotr quality factor
Post by buddypoor on Oct 23^rd, 2008, 12:21am

1.) The quality factor you have mentioned is defined with respect to the parasitic phase shift introducd by the amplifier - for example plus two degrees.
In this case the oscillating frequency is shifted until the passive network introduces minus two degrees in order to fulfill the oscillation condition.
When the phase slope is large the amplifier phase shift effect causes only a small frequency shift.
This explains the proportionality between the phase slope and the definition of the so called "quality factor".
2.) However, this quality factor applies ONLY to oscillators which consist of frequency selective networks and an amplifier (two-port oscillators); thus, it applies NOT for negative resistance oscillators as well as integrator loops.
3.) Therefore, this factor has only a limited value and cannot be used to compare two-port oscillators with other types like neg. R or integrator type oscillators.

Title: Re: oscillaotr quality factor
Post by raja.cedt on Oct 23^rd, 2008, 6:41am

Title: Re: oscillaotr quality factor
Post by buddypoor on Oct 23^rd, 2008, 8:21am

raja.cedt wrote on Oct 23^rd, 2008, 6:41am:

hi buddypoor,
thanks for your ans.but i didn't get why Q will applicable for only two port and why not -ve resistance oscillator. I think we can treat every oscillator as -ve resistance or you can treat amp with frequency selective network EX colpits. can u tell me this one.
Another question is there any chance of getting -ve Q?

1.) OK, let´s formulate it with other words: If you speak about phase slope - what do you mean ? Phase slope of which unit ? It only makes sense if you can devide the oscillator in two parts: one frequency selective part (with a phase slope) and another part which is desired to deliver constant gain (ideal amplifier). Where and how could you measure phase slope of an one-port oscillator ?
2.) What do you mean with -ve Q ???

Title: Re: oscillaotr quality factor
Post by raja.cedt on Oct 27^th, 2008, 9:51pm

hi buddypoor,
Thanks for your reply.Does quality factor discussion is valid for all systems or only second order systems?

Thank you.