Alternatively, when looking at the frequency response, the gain may rise near the upper end of the passband, which is usually undesireable. In an extreme case the gain can become high enough to cause oscillation, that is, a sustained cyclic response without any input. This effect generally renders the system useless or even destructive. The stability problem is inevitable. It results from the fact that the feedback, which is connected so as to be negative at low frequencies, usually becomes positive at high frequencies.
Good stability is usually possible provided the loop gain is low enough.
The main reason the feedback ultimately becomes positive as the frequency increases is that both the control system and the load it is driving contain components that can store energy. Capacitance and inductance are electrical energy storage elements, and mass and springs and raising an object against gravity are mechanical energy storage elements. Since the drive to physical devices is not infinite, the response must dwindle toward zero as the frequency approaches infinity, with an associated phase shift approaching 90 o . Several phase shifts can add up so that the total around the loop equals 360 o , which is positive feedback. Only 180 o of additional shift from the energy storage elements is needed to cause positive feedback, since the connection at the comparator introduces 180 o to make the feedback negative at low frequencies.