Discussion
A system with gain margin close to unity or a phase margin close to zero is :
Answer: C
Such a system is oscillatory.
Gain margin is how much the loop gain can increase before instability, and phase margin is how much extra phase lag can be tolerated. A gain margin close to unity (0 dB) or a phase margin close to zero means the open-loop frequency response passes very near the critical point -1 in the Nyquist plot. The closed-loop system then has poles close to the imaginary axis, so its response is lightly damped and oscillates for a long time; at exactly unity gain margin or zero phase margin it oscillates continuously.
A highly or relatively stable system has comfortable margins, typically a gain margin of several dB and a phase margin of 30-60°. An unstable system has a negative margin.
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