Patent GB-869808-A (GB)
Title: Improvements in or relating to radar systems
Inventor: FIOCCO GIORGIO DOMENICO
IPC: G01S13/60
Filed: 1958-12-01
Abstract: 869,808. Radar. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. Aug. 27, 1959 [Dec. 1, 1958], No. 38739/58. Class 40(7) In a frequency modulated C.W. radar system the phase relationship between a given harmonic of the modulation signal and the same harmonic of the signal derived by mixing the transmitted and reflected waves is utilized to determine the range of the reflecting target. The Specification includes a mathematical analysis of the system and shows that the signal produced by mixing the modulation signal with the derived signal consists of a Bessel coefficient term and a cosine term, the location of the (phase-dependent) nulls of the cosine term being determined by the phase of the modulation signal with respect to that of the derived signal. The amount of phase shift which must be introduced into the modulation signal to reduce to zero the amplitude of the signal produced by mixing it with the derived signal is then a measure of the target range. Fig. 2 shows one embodiment in which frequency modulated waves produced by frequency modulating a micro wave source 2 by a sinusoidal, source 1, are radiated by a directional aerial 3 and after reflection from a target, received by an aerial 4 and combined in a mixer 5 with a portion of the transmitted energy (derived via a coupling 6) to produce a low frequency signal consisting of a spectrum of frequency bands centred on harmonics of the frequency of the modulating source 1 and each composed of two side bands separated by twice the Doppler shift frequency (due to relative motion between the system and the target). A predetermined one of these harmonic bands is selected by a filter-amplifier 7 which supplies a double side-band suppressed carrier signal to a detector circuit comprising a mixer 8 in which it is combined with a signal of the same predetermined harmonic derived from source 1 via a harmonic generator 9 and an adjustable phase shifting device 11. The output of mixer 8 consists of a frequency dependent on the Doppler shift and this signal may be supplied over lead 10 to known arrangements for velocity measurement. For the measurement of range, however, and in accordance with the theory of the system outlined above, the phase shifter 11 is adjusted (e.g. manually, via handle 12) until the output from mixer 8, as indicated on a meter 13, falls to zero. The amount of adjustment which this requires is then a measure of the range of the target and the scale associated with the handle 12 may therefore be calibrated in terms of range. Instead of manual adjustment of 12 the meter 13 may be arranged to adjust the phase shifter 11 automatically via a mechanical link to maintain the output from mixer 8 at zero level (Fig. 3, not shown). To enable velocity measurement to be effected at the same time as range measurement (the signal on line 10 being zero) a further mixer supplied from filter amplifier 7 and from phase shifter 11 via a fixed 90 degrees phase shifter is provided (Fig. 4, not shown). An arrangement for the visual display of the phase difference between the output signals of the filter amplifier 7 and harmonic generator 9 comprises, as shown in Fig. 5, additional mixing circuits 15, 16, in which the two output signals from 9 and 7 are mixed with the output of an oscillator 17 operating at a frequency which differs slightly from that of the outputs of 7 and 9. The outputs of the mixers 15, 16 are then supplied via L.P.F.'s 18, 19 to perpendicular pairs of deflecting plates of a cathode-ray tube 20 and produce an elliptical trace from which the phase-difference, and hence the range, may be evaluated by known means. When the total range of the system is large (i.e. more than the distance represented by two successive null values of the cosine term mentioned above) the arrangements so far described give ambiguous results and to avoid this the modification shown in Fig. 6 is employed. In this arrangement two non-harmonically related harmonics of the source 1 are produced by a third harmonic generator 39 and a fifth harmonic generator 59 and supplied via adjustable phase shifters 311, 511 to respective mixers 38, 58 the other inputs to which consist of the third and fifth harmonics of the output of mixer 5 derived via band-pass amplifiers 37, 57. The output of mixer 58 is supplied to the amplitude responsive device 513 which adjusts phase shifter 511 to reduce such output to zero and also drives, via a 3/5 ratio gear box 35, one input shaft S 1  of a differential gear D a second input shaft S 2  of which is driven by the mechanical linkage which, under the control of an amplitude responsive device 313, adjusts phase shifter 311 to reduce the output of mixer 38 to zero. The output shaft S 3  of the differential gear then indicates (pointer C) the sub-range of the target and the pointer F the position of the target in that sub-range. Instead of deriving the two non-harmonically related signals from the single modulating source such as 1, two separate modulating sources of appropriate frequency may be employed.