Dear Colleagues,
I want to ask about the reasons for obtaining always positive magnitude in dB and phase always greater than -180 in Bode plots when checking the stability by linear analysis tool in Simulink and what this indicates about the system stability?
I am grateful in advance.
At first, we should keep in mind that - when discussing stability in the Bode diagram - we speak about the LOOP GAIN of a system with feedback.
When - at a certain frequency - the phase of the loop gain reaches -360 deg (identical to 0 deg) and the gain is still above 0 dB, the stability criterion is violated and the closed sysytem will be unstable.
In this case, we will have positive feedback with a loop gain above unity.
Note that the above mentioned 360deg criterion assumes that the phase inversion at the summing node is included in the total phase shift (as is the case for real measurements/simulations).
However, when the minus sign at the summing node is not included (may be the case for some calculations) the "critical" phase is at -180deg.