Control applications are implemented using real-time operating systems. Digital control theory is based on sampling intervals that have to be strictly met in order to get predictable behaviors. However, a real-time system may introduce execution jitters that may cause unpredictable effects on the control application. Stability, overshoot and settling time may be affected when an inadequate real-time system is used. Several papers have proposed different mechanisms to measure the jitter that a real time system produces. However, jitter can not be translated as a performance criterion in control theory. On the other hand, frequency domain techniques are widely applied in control theory for designing control strategies, as well as analyzing and measuring the performance of control mechanisms. In this paper, frequency domain analysis is used to measure the perturbations that a real-time operating system may produce on a control application. Harmonic distortion is defined as a criterion to evaluate the control performance of the the application. Experiments show that higher priority tasks are likely to be used for control tasks since the perturbation produced up to an utilization factor of 70% is adequate for most control applications.