Swept sine waves, white random noise, and the Dirac impulse function all provide flat responses and all are available in arbitrary waveform generators. Keeping in mind that the impulse function is the ...

Norman R. Swanson, Clive W. J. Granger, Impulse Response Functions Based on a Causal Approach to Residual Orthogonalization in Vector Autoregressions, Journal of the American Statistical Association, ...