Hi,

I understand this assertion. But i'm actually not understand why a system would be stable if a disturbance goes around the loop and come back to the same point, with a higher magnitude, and a different phase ? If the gain is inferior to 1, whatever is the phase, i can understand that the disturbance will diseaper after passing several times in the loop... Please tell me where is my error (I know from the transfer function that it would be unstable if GH is equal to -1, which correspond to a gain = 0 dB and a phase equal to -180 °)

Suppose the gain GH is higher than 1 and equal to G and the phase shift is equal to P and is not equal to -180 ° at the frequency f. I have a disturbance which occurs after the summing node. The disturbance is equal to Asin(wt) with w = 2pi*f. At the same point (after the summing node), the disturbance would be equal to G*Asin(wt + P -180). So now we have a disturbance which is higher than the previous... and it is just shifted in phase from the previous...

Thank you very much

I understand this assertion. But i'm actually not understand why a system would be stable if a disturbance goes around the loop and come back to the same point, with a higher magnitude, and a different phase ? If the gain is inferior to 1, whatever is the phase, i can understand that the disturbance will diseaper after passing several times in the loop... Please tell me where is my error (I know from the transfer function that it would be unstable if GH is equal to -1, which correspond to a gain = 0 dB and a phase equal to -180 °)

Suppose the gain GH is higher than 1 and equal to G and the phase shift is equal to P and is not equal to -180 ° at the frequency f. I have a disturbance which occurs after the summing node. The disturbance is equal to Asin(wt) with w = 2pi*f. At the same point (after the summing node), the disturbance would be equal to G*Asin(wt + P -180). So now we have a disturbance which is higher than the previous... and it is just shifted in phase from the previous...

Thank you very much

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