lawfulgm
Junior Member level 1
Hi all, I have some question regarding the stability of a feedback system.
First figure shows closed loop AC frequency response (green), and feedback loop gain (purple), and feedback phase response (red).
From the first figure, at 710.97 Mhz, the loop phase response is 180' shifted from the starting phase (-180). At this frequency, the loop gain is much larger than 1 (22.7 dB)
However, from the step response, which is attached in the second figure, I am not experiencing any oscillation or instability problem.
It shows large over/undershoot but I think that is due to the low phase margin at 11.4 Ghz (only 28 ' of phase margin)
I understand that Barkhausen's criteria is a necessary condition but not a sufficient condition. However, this circuit has complex poles at higher frequency even when the loop gain is larger than 0dB, and I think it has enough sufficient condition to be oscillating.
If anyone has a good explanation on this phenomenon, please share you knowledge.
Thanks,