Abstract :
It is shown that, whatever the shape of the clutter spectrum and whatever the order of a binomially weighted canceller, the output clutter residue is minimised if the local oscillator is locked to null the interpulse phase shift just prior to the final subtraction. In the presence of receiver noise, however, this phase shift will become contaminated. Simply to filter out this phase noise would reduce the ability of the system to adapt to variations in the mean clutter velocity. An alternative approach is to derive the locking signal from a phase detector placed further back along the canceller. The losses in cancellation ratio consequent upon doing this are derived in general terms, and it is shown that the losses are dependent almost entirely upon the shape, and not the scale, of the clutter spectrum. Consideration of two hypothetical and two realistic model spectra possessing considerable asymmetry suggests that, for cancellers of up to the third order, the additional losses are not large. A similar approach is used to derive the losses for a general class of canceller when the locking is effected by nulling the mean interpulse phase shift of the input.
