Hi praveen,

If your AC sims at different operating points indicate an osc of 153MHz, then why r u saying that it is 130MHz as in yr original post ?

Or are you saying your TRANSIENT sims at various dc operating points are around 153MHz ? If so, can you run an AC sim and determine the frequent when PM is 0?

... =)

Hi RAJ, regarding your last posting:

What i feel is if any system has clossed loop gain>1 and phase is 0, then it start blowing up and if there any non-linerity eventually it will make your gain =1 and this what i feel is criterion for oscilations. Please corect me if am wrong.

i didn't understand your post regaring some thing missing in Barkhausen criterion. Could you please explain  this.

At first, two definitions (to refresh):
* If a circuit oscillates it is necessary that the loop gain T(s) is unity (T=1); that is the (necessary) Barkhausen condition;
* A sufficient condition for oscillation would be: A circuit always will oscillate and produce a sinusoidal output if ....? According to my knowledge such a condition has been not yet formulated.
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Correction (typing error?): The loop gain T(s) must be equal or larger than 1 (not the closed-loop gain) to start oscillations. What do you mean with "blowing up"?.

Regarding "your criterion" for oscillation: There are many loop gain expressions and also many circuits that exhibit a loop gain that is real and equals unity at one single frequency WITHOUT being able to oscillate.
Examples (counter example to the Barkhausen criterion):
1.)  T(s)=-N(s)/D(s) with N(s)=9(26+2s+s^2) and N(s)=s(s+1)(s+2).
One can show that T(s)=1 at app. 0.7 Hz.
2.)  Classical WIEN oscillator with exchanged opamp inputs (opamp ideal). The loop gain will still be T(s)=1, however, the circuit never will oscillate. Instead, the circuit will exhibit latch-up.
__________________________
One final question:
What is the meaning of your posting:
is it posb to find oscilation frequency of any REAL sytem  with ac sim? Up to my knoledge only in case of  two integrators in loop it's posible.    

I don`t understand the restriction to two-integrator circuits.

hello RAJ,

What you are saying there is sufficent condition for the sustained oscilation. What i feel is if you have gain>1, then it start increasing with time [i](exponentially this is what i mean blowing) and if you have sufficient non-linerity in the ckt then it will be oscilate[/i].

No, that`s not the case. Although most of the circuits with loop gain T(s)=1 will behave like this, there are exceptions as given by me as counterexamples. Thus, this criterion is NOT sufficient. For example, the WIEN circuit with opamp inputs exchanged needs a RISING gain for rising amplitudes in order to bring the eigenvalue back to the LHP of the s-plane (stabilizatuion). But that is contradictory.

Coming to your example WIEN oscillator with exachanged opamp inputs, it can't oscilate because there is no DC -ve feedback, how it will get bised?
That`s the reason I mentioned the IDEAL opamp that needs no dc bias.
Nevertheless, it will not oscillate.

Finally, believe me, up to now there is no sufficient oscillation criterion.
If you have the opportunity to read "Analog Integrated Circuits and signal processing" you will find some recently published articles dealing with this subject.
Regards
buddypoor

Oh, sorry - I forgot to mention that
"Analog Integrated Circuits and signal processing"
is a monthly magazine (Springer.com).
It contains very interesting contributions from authors round the world.

I wonder, if any member of this forum knows something about a sufficient criterion for a harmonic oscillator

Thanks
buddypoor

Hi Raja,
i am not very clear about ur point saying "if u have 0 deg phase shift and gain >1, then the signal will blow up and finally the ckt non-linearities will limit". This means that even though the small signal gain is >1, the large signal gain will become one.

Can u explain what happens in PLL, where its low freq gain is >> 1 and phase shift is 0 deg, but it is not unstable. We only bother about the freq at which the loop gain goes exactly 0 deg and see if the phase margin is ok there.

hello ,
PLL is diferent case.

let me explain two wings in stability discussion. I am talking about all pole system, for this if you have >1 gain and phase=180deg you would end up with blow but for a system which has zero's we could use nyquist (after all bode is one special result of nyquist plot ). Please find the attached fig for more details.

@rajkumar: for pll please draw nyquist or root locus, but forunatly bode is working.

Thanks,
Raj.

Yes, I agree with raj.
The system belonging to the green curve will be stable after closing the loop - it is a system called "conditionally stable". It will become unstable if the gain falls below a certain limit.

Hi RAJ,

I suppose, you mean that ringing occurs after closing the loop with a negative sign around the given TF? Correct?

This is another example that in some cases the concept of stability margin cannot be applied and the Nyquist plot must be used to decide stable/unstable.
Comment/Addendum:
For clarification - both margins are positive which means the loop will be stable after closing. However, the amount of phase margin gives not the correct picture about stability margin (the gain margin is very small). That means: The concept of stability margin can be applied to decide stable yes/not but the PM indicates a false safety.