Abstract: An apparatus for characterising an input signal within abroad frequency band by comparing the same input signal in a plurality of channels, in order to operate digital Electronic Support Measures (ESM) which require a broad bandwidth to function. The apparatus comprises one or more signal input bands spread across the broad frequency band, a means of splitting the input signal in each input band into a plurality of separate channels, and a means of sampling each channel, wherein the sampling means in each channel runs at a different clock rate from sampling means in each of the other channels within the input bands, so as to remove the ambiguities inherent in frequency aliasing.