Hi jugemu1234,

At first, the stability margin certainly will NOT be app. 150 deg.
Just the opposite is true: The rapid increase of the phase function around 150 kHz indicates stability problems (worst case: instability!).
The reason is as follows: The concept of phase margin is applicable only in case of a loop gain function which has no zeros and which is a minimum phase system - and the increase around 150 kHz looks like a zero influence. (By the way: I assume you have shown us the loop gain, did you?)
More than that - you cannot trust the results of an ac analysis alone, because even in case of instability you can get a curve which looks more or less good.  
Regards

Addendum/Correction: Because of the rapid phase increase it is more likely that the whole system is unstable.

@ maynk: i guess what he wants you say is  even in case instability also you will get pretty nice ac wave form, so don't believe bode plots in case both magnitude and phase plots are monotonic.

@jugemu1234:hi man in your ac plot i saw sudden change in phase by almost 180..this is some times  common mistake. How  did you open the loop because if your both L and C are small then at resonance frequency you could see this type of weird behavior, so first you increase L and C and see whether you are getting same rapid change at that point.

Thanks,
rajasekhar..

Mayank wrote on Nov 1^st, 2009, 10:09pm:


Hi MAYANK !
1) For example, when you by mistake connects the feedback line to the non-inverting opamp input (that means: positive feedback) the result of the ac analysis looks normal. That is because the program does find an operating point (which you also can calculate by hand, but it is unstable). However, the program does not know that it will be unstable because it does not take care about noise and power switch-on.

As another example, take a non-compensated opamp and use 100% feedback. The opamp in reality will be unstable. But you will see that the result of ac analysis - as far as the magnitude is concerned !! - looks normal. However, the phase will exhibit a rapid increase - as in your case!! This is an indication for instability !

2.) The answer to your second question cannot be given in short here   in the forum. Please refer to the complete NYQUIST stability criterion. In this context, you should realize that the stability check in the BODE plot is nothing else than a transfer of the NYQUIST criterion from the complex plane to the simpler BODE plot. And according to NYQUIST the parameter called "phase margin" indicates stability/instability not for all kinds of functions. Description and explanation of this criterion can be found in books on control theory.    
Regards

hi,
   thanks for your quick cheek.I wounder how you are getting 180 phase change at one point..the reason could be some complex pole, but how in open loop you get complex pole? any how i donno much may be some senior person should help us..have you done transient simulation?

Thanks,
rajasekhar.

Hi,

Pls find attached. This is simplified model of whole loop and how I made it open loop. "ACBLK" indicates the circuit I had attached previously. The very first Bode of this thread is not coming from this circuit but the picture attached yesterday (on which I noted 150 PM) is derived with this open loop. And AC source is on Vref.

Thanks,

buddypoor wrote on Nov 2^nd, 2009, 2:23am:

Hi,
     Buddy, thanx for your reply....
      Jugemu, I agree with buddy that if you want to measure your open Loop Gain response, why are you placing an extra LC ckt in between...Do the normal return ratio analysis by breaking the loop at resistive divider node & givng an ac signal to +ve node of opamp & measuring the tf upto the loop-breaking point...I guess that should do...

 You can also place an i-probe available in spectre b/w resistive divider node & +ve opamp i/p and do a stb analysis to obtain LG response...

--Mayank.

Hello raja,
               Yeah, return ratio method is for hand-calculation....My second suggestion refers to middlebrook method only....
       Placing an i-probe or a CMDM probe provided in analogLib of spectre virtuoso is an element for middlebrook FB analysis...
My point is to avoid placing a LC filter,which imitates simulation conditions for return ratio analysis,(instead do a hand calculation by return ratio method)  &  use the middlebrook method for simulation, (which places baluns/ideal transformers and does a stability analysis).
     Instead spectre + virtuoso makes life easy by providing an iprobe/ cmdm probe cellview which you can directly place in your schematic and do the stb analysis on it.....There you get your LG without placing any L,C ckt inside your schematic.....

--Mayank