GB990680A - Improvements in noise cancellers for signal processing systems - Google Patents

Abstract

990,680. Radio receiving systems; radar; aerials. GENERAL ELECTRIC CO. June 2, 1961, No. 20084/61. Headings H4A, H4D and H4L. A noise canceller for a signal processing system comprises a main signal transmission channel and an auxiliary signal transmission channel, means for adjusting the amplitude and phase of interference signals in the auxiliary channel to equal the amplitude and phase of interference in the main channel, and means for subtracting the adjusted signals in the auxiliary channel from the signals in the main channel. Fig. 1a shows a circuit for reducing the effect of noise jamming in a radar receiver. Signals from a directional aerial 10 are mixed with a local oscillation in mixer 14 and passed through a subtraction circuit 22 and a bandpass filter 23 to an output terminal. Jamming signals from a non-directional aerial 12 are mixed in unit 18 with a local oscillation offset from that of the main channel. One output of mixer 18 is fed through a band-pass filter 27 to a mixer 24 where it is mixed with the output of the main channel; the output of mixer 24 is passed through a narrow band filter 26 to provide a signal at a frequency equal to the difference between the two intermediate frequencies and having an amplitude proportional to the average power of the jammer. The phase of this signal is adjusted to that of the main channel by mixing it with a second output of mixer 18 in mixer 28. The output of mixer 28 is introduced to the radar I.F. channel through subtraction circuit 22 forming a non-linear closed loop providing automatic gain adjustment so that the residual jamming signal approaches zero in the radar I.F. channel. The speed of response of the system is dependent on the loop gain and the bandwidth of filter 26. Since loop gain is proportional to average power the loop cancels jamming signals which have high average power, while it is relatively insensitive to echoes which have low average power. In a modification, Fig. 2 (not shown), aerial 12 is replaced by three non- directional aerials equally spaced in a linear array for side lobe cancelling. The outputs of the aerials are respectively mixed with local oscillations, the frequencies of which form an arithmetical progression, and the sum of the outputs of the mixers replaces the output of mixer 18 in Fig. 1a. The filters 23 and 27 are omitted and filter 26 is replaced by three narrow band filters in parallel. The pattern of the non-directional array is caused to scan by phasing of array outputs.