pancho_hideboo

Hi.
See attached figures. These are Bode Diagram and Nyquist Diagram of conditionally stable system.
In Bode Diagram, Phase Margin at gain crossing point(green point) is positive. So as far as seeing phase margin, this system is stable. On the other hand, phase is across 180deg at red, yellow and blue points. Although gain is smaller than 0dB at blue point, but gain is larger than 0dB between red and yellow points. I understand this system is stable at blue point. But why is this system stable between red and yellow point ?
While in Nyquist Diagram, locus doesn't enclose a point of 1+j0, so we could conclude this system is stable. Also a root locus shows this system is stable.
In general the system which gain is larger than 0dB at phase crossing point(180deg) could be unstable.
How should I interpret this bode diagram ? :/
