GB962440A - Radar system - Google Patents

Abstract

962,440. Radar. COMPAGNIE FRANCAISE THOMSON-HOUSTON. Feb. 9, 1961 [Feb. 12, 1960], No. 4906/61, Heading H4D. Relates to a pulse radar for suppressing echoes from stationary objects in which signals of two different frequencies F -f and F+f are transmitted simultaneously and are each received in individual channels where they are converted to signals of frequency f, the outputs from the two channels being combined, e.g. in a synchronous detector, to give a phase-sensitive output which is applied directly and through a delay device 37 to a subtraction arrangement 33 to suppress echoes from fixed objects. According to the present invention the necessity for rigid stabilization of the transmitted frequencies in such a system is avoided by including in at least one receiving channel a further frequency converter to which is fed a local signal of frequency f1. A shown, the transmitted frequencies F -f and F+f are produced by mixing a frequency F from an oscillator 2 with a frequency f produced in a frequency multiplier 23 from a subharmonic input f/n from an oscillator 24. The corresponding echo signals in the receiving channels 25 and 26 are each converted to frequency f by mixing with the frequency F and are then mixed in frequency changers 27 and 28 respectively with frequencies f2 and f1 derived from the subharmonic f/n by frequency multipliers 31 and 32 to give equal frequency outputs f-f2 and f 1 - f (i.e. f-f2 = f1-f, f1+f2=2f) which are applied to a synchronous detector 21 coupled to the cancellation circuit 22; alternatively, the frequencies f1 and f2 may be such that f1 - f2 = 2f, the equal output frequencies from channels 52 and 26 then being f+f2 and f1-f. In a modification, Fig. 3 (not shown), the frequency change 27 is omitted and the output frequencies f and f1-f from the channels 25 and 26 are applied to a mixer whose output is applied to the cancellation circuit 22. In another modification. Fig. 4 (not shown), the frequency changer 27 is omitted in channel 25 and in channel 26 the output f1-f from the frequency changer 28 is mixed in a further frequency changer with the frequency f2 to give an output f.