Hi!

I think that your phase margin is good but I the problem is that you also need to take a look to the gain margin, that should be negative (in terms of dB). In your case, you do not have gain margin, so I think that your circuit will be unstable as if you had a poor phase margin.

Bye!

Monte

dandelion wrote on Jun 20^th, 2007, 6:11pm:

I am not so sure ... I think one needs to apply the extended nyquist stability
criteria. And maybe this system might even be stable..?...
But I am not so familiar with that, because usually it is enough
to check the phase-margin.

But probably somebody else has more experience.?..

One thing to be careful of in this case is slew-rate limiting. When slewing the gain of the amplifier will drop and could cause the feedback system to become unstable. I have seen this before. In my case it manifested itself as a circuit that was stable for small signals, but as soon as an abrupt change occurred in the input signal the circuit began to oscillate and would not stop. I don't know that that is what will happen in this case, but I suggest that you explore the possibility.

-Ken

Hallo ,

the circuit is clear to be unstable, the range of frequencies whose phase margin is below 45 degrees will lead to oscillation.

In another way, any system that has more than one pole, has a problem at the second pole where the feedback of the system will turn to be positive and this will lead to oscillation, the solution is one of two:
1. To damp the signals at these frequencies by shifting this pole far to high frequencies where your circuit or amplifier has at least -40dB, cancelling these signals before having this positive feedback effect.
2. The second thing is to compensate this second pole, so that you bring a zero at the same frequency of the second pole to cancel its effect early before it (the second pole) brings the phase margin to critical values.

In your case at the range of frequencies below the 45 degrees you have still a gain, this means that the signal will oscillate free plus amplification, which is the unwanted case.

Best regards,

aamar

Hallo Frank,

I accept your answers no problems, the two poles could be on the left hand side, and even the zero, but I was replying to the original question where dandelion said

"At the point A, the phase obviously reaches -180'C while the gain is larger than one, what's the impact to the performance at thsi freq?"

and I think he means the step response nothing else or at least what will happen if you applied a signal with exactly this frequency to the system.

Any how it is good to mention that the problem can have a different face if we look to it in another domain.

Thanks and best regards,

aamar

aamar wrote on Jun 22^nd, 2007, 2:01am:


yes, just as aamar said, what I mean is i wonder if the step response or other injection will induce ring even they will not oscillate eventurally.

Here lead to my another question. In many design spec, it will always give the phase margin spec, e.g., it is needed to be larger than ~60 degrees. I want to know, here, if the phase margin >60'C is needed to guarantee the circuit will not oscillate by itsself eventuarlly?. Or 60'C is needed to guarantee good setting behavior, i.e., 60'C can have good step response which has no rings?

I found the word "stabilty" made many people puzzle.

So, would you pls. give me some commnets?

Thanks

Dandelion,

If you look at the openloop gain and phase of a second order PLL, you'll see a similar situation: high gain and 180 deg phase. With the zero in the right place as you have, your feedback loop will be perfectly stable.

If in doubt, check the closed loop transient response.

-Monte