On the maximum rate of fluctuation in mode-stirred reverberation

Theoretical limits are derived for the maximum permissible rate of mechanical/electronic stirring or spatial/spectral scanning inside a reverberant cavity. These limits are based on upper bounds for the maximum rate of field fluctuations and are obtained by imposing the requirement of quasi-stationarity on the interior cavity field. For a sinusoidal excitation field, the interior field is represented as a narrowband random hybrid amplitude-plus-frequency modulation, as induced by the stirring or scanning process, and is based on an analytic field formulation. Distortion (nonlinearity) of the modal and effective field is quantified for first-order variations of amplitude and frequency. Its dependence on the random amplitude modulation index is demonstrated. The latter is estimated from macroscopic system parameters. The effect of compensation of net input power on the distortion and maximum stirring rate is analyzed. The maximum stirring rate exhibits an inverse power law dependence on the operating frequency. For a specified level of maximum distortion, maximum rates of electronic, mechanical, or electromechanical mode stirring are derived. The effect of the order of the EUT transfer function on the distortion, as well as the detuning, bandwidth, and amplitude distortion of the perceived test field caused by stirring or scanning are quantified and analyzed.