Sidelobe rejection filter

Abstract

Disclosed is a Sidelobe Rejection Filter for reducing the time sidelobes seen in the auto-correlation output signals from a matched filter. Such matched filters are used with radars utilizing pulse compression techniques, for example. The sidelobe rejection filter may be utilized whenever the time response of the auto-correlation signals includes a plurality of sidelobes each having a period .tau. and having essentially the same amplitude. Accordingly, the disclosed sidelobe rejection filter is particularly useful for pulse compression radars employing linear FM (i.e., Chirp) or Barker phase coded waveforms. The matched filter includes a delay circuit coupled to receive the time response of the auto-correlation signal, which delay circuit inserts a delay of .tau., and a subtraction circuit for finding the difference between the delayed and undelayed time FM pulse compression techniques, the output of the subtraction circuit need merely be rectified to obtain an output signal whose sidelobes are substantially reduced in amplitude. In the case of phase coded pulse compression techniques using Barker codes, on the other hand, the output of the subtraction circuit is supplied to both positive and negative rectification circuits, one of whose outputs is then delayed by N.tau., where N is the number of bits in the Barker code. The output of the N.tau. delay circuit is well as the output of the other rectification circuit is then applied to an adder for further reducing time sidelobes. The output of the adder is applied to yet another rectification circuit for providing an auto-correlation response which is substantially free of time sidelobes.