Wideband correlation-like operations using systems with time-varying coefficients

A new alternative to cross-correlation is examined which forms the basis for developing analogue hardware systems designed to operate in the GHz range, beyond the point where digital spectral methods are realistic. The methodology is designed to realise the time resolution and noise rejection capabilities of wideband modulation by chirps, PRBS or wavelet functions when combined with cross-correlation detection, but without the need for modulation. This approach should be of special relevance for optical radar applications where technical limitations of modulators make it difficult to take advantage of the benefits of wideband modulation schemes. The technique offers a considerable increase in the time resolution available from conventional cross-correlation using unmodulated signals. The approach is based upon parametric control of the impulse response of filters with time-varying coefficients which transform the bandwidth of the input signals and exploits recent developments in the theory of the frequency response functions of such systems. There can be no exact analogue between time-varying coefficient systems and their more traditional constant coefficient counterparts, however the CTR filter has some similarities with an ensemble of constant coefficient functions embedded in a correlation architecture and with a wideband coherent detector.