Hello all,
My question is best illustrated with the below example I am seeing for my system:
At the GBW, the phase response is 55deg from the 180deg pt. This would imply a phase margin (PM) of 55deg.
However: At a frequency < GBW, my phase response dips to 40deg (LHP zero present).
=> Is the PM = 55deg or 40deg?
I see by definition PM is measured at GBW implying PM = 55deg. If so, can someone please explain the rationale behind
PM not being 40deg for this scenario i.e. why would the 40deg frequency not cause more ringing?
Also, gain margin (GM). Is GM really that useful?
In going from open loop to closed loop stability analysis, it seems that PM bridges the two i.e. dampening
ratio is related to PM (PM/100 ~ zeta for some cases).
If this is completely true, what really is the use of GM? Should one even bother calculating it?
Thanks,
Diarmuid