GB1106656A - Improvements in automatic radar tracking systems - Google Patents

Abstract

1,106,656. Frequency modulated radar. COMPAGNIE FRANCAISE THOMSON HOUSTON-HOTCHKISS BRANDT. 15 July, 1965 [17 July, 1964], No. 30170/65. Heading H4D. In a frequency modulated radar system, radiation is transmitted starting at time t<SP>1</SP>=0, with an angular frequency given by #(t)= # 0 + αt<SP>1</SP> (see Fig. 1b) and, after a sound trip delay of #, an echo signal Ei is received with an angular frequency given by #(t) = # 0 + α(t<SP>1</SP> - #). The echo signal thus has the form A gating signal is applied to a gate 2, receiving the echo-signal at the output of gain controlled input amplifier 1, having a duration T and commencing at a time (# + t 0 ) supposedly synchronous with the commencement of the echo signal (i.e. it is assumed that t 0 =0). The gated echo signal is fed via a " total " signal path 20 and a " difference " signal path 10, to respective mixers 21 and 11. An oscillator 50 is triggered by the gating signal to generate a signal having an angular frequency given by # 0 +# 0 + α(t<SP>1</SP> - 0 - t 0 ). This signal is applied to mixer 21, to produce a " total " beat signal, Fig. 1b, having a duration # and an angular frequency of (# 0 - αt 0 ). The oscillator signal is also applied to a phase reverser 51, which reverses the phase of the oscillation after a time #/2 and applies its output to mixer 11. The output of mixer 21 is applied to a narrow band filter 23, centred on frequency # 0 , to give a signal which is applied to one input of phase demodulator 3. The " difference " beat signal output of mixer 11 is applied to a similar narrow band filter 13, to give, from two signals corresponding to the two halves of the phase reverser output, a signal After passage through a 90 degrees phase shifter 14, this signal becomes and is applied to the second input of demodulator 3. The demodulator gives an output of the form:- or with reference to standard amplitude (A#/2)<SP>2</SP> and assuming t 0 is very small:- Thus this signal is a linear function of the error timing to of the gating signal, and by making α, #/4 large, a very sensitive error signal can be produced for correcting the position of the gating signal. The detected output of filter 23 may be compared with a reference signal to produce a gain control signal for amplifier 1 whereby the amplitude of the signal # is kept constant. By replacing filters 13 and 23 by respective groups of parallel connected subfilters, having centre frequencies staggered either side of # 0 , feeding the outputs of corresponding ones of the sub-filters to respective phase demodulators and adding staggered biases to the outputs of the demodulators (see Fig. 2) the waveforms of Fig. 3a produced by each demodulator, may have their linear (t 0 # 0) portions aligned on a composite line, Fig. 3c, to give a much extended linear portion over which the equation E = αt 0 #/4 holds.