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Active Disturbance Rejection is a relatively new, and quite different, design concept that shows much promise in obtaining consistent response in an industrial control system full of uncertainties. But most of the development and analysis has previously been shown in the time-domain. In this paper, frequency-domain analysis of such a control system is performed to quantify its performance and stability characteristics. The transfer function description of the controller is derived and, together with a highly uncertain linear time-invariant plant, the loop gain frequency response is analyzed. The result shows that the active disturbance rejection based control system possesses a high level of robustness. The bandwidth and stability margins, in particular, are nearly unchanged as the plant parameters vary significantly; so is the sensitivity to input disturbance. Such characteristics makes this control system an appealing solution in dealing with real world control problems where uncertainties abound.