Question

why gain margin is calculated at phase cross over frequency

Answer 1

In other words, the gain margin is 1/g if g is the gain at the –180° phase frequency. Similarly, the phase margin is the difference between the phase of the response and –180° when the loop gain is 1.0. The frequency Wpm at which the magnitude is 1.0 is called the unity-gain frequency or gain crossover frequency.

Answer 2

Now, assuming your “phase crossover frequency” is the one where your phase is crossing -180*, even in this case, there can be only one.

HOPE IT WILL HELP U DEAR ♥️