Patent GB-967083-A (GB)
Title: Improvements in or relating to radar systems
Inventor: AGAR WILLIAM OLIVER
IPC: G01S13/22
Filed: 1961-08-04
Abstract: 967,083. Radar. MARCONI CO. Ltd. May 21, 1962 [Aug. 4, 1961], No. 28388/61. Heading H4D. Relates to a C.W. radar system of the kind in which the transmitted waves are modulated in phase or frequency by a modulating frequency which is so adjusted that the modulation on the received echo signal is in a predetermined phase relationship with the modulation on the transmitted waves. According to the invention there are provided a plurality of variable different - frequency modulating sources in predetermined frequency relationships, means enabling those sources to be applied in succession to modulate the transmitted waves and means, controlled by received signals and operable when any one source is applied to modulate the transmitted waves, for automatically adjusting that source to that synchronous ranging frequency which is nearest to the frequency of the source before such adjustment is effected. In the embodiment C.W. microwave energy from unit X2 is phase modulated by a frequency fed over line ML; transmitted energy is leaked to receiver mixer X5, the output from which is fed to filters X9, X10. For example if the limits of modulating frequency are 0.7 and 1.6 Mc/s. and the maximum Doppler shift expected is 0.1 Mc/s., X9 is a bandpass filter covering 0.6 to 1.7 Mc/s. and X10 is a low-pass filter covering 0 to 0.1 Mc/s. Filter X9 output is applied to a mixer X11 fed also with frequency F via a pre-set phase adjuster X14; the differencefrequency output is fed to one input of a phase sensitive detector X12 affording a D.C. output of polarity dependent on the phase relation between that one input and the output of filter X13 after 90 degrees phase shift in unit X13. The D.C. is utilized to control the frequencies F# 1 , F# 2  . . . F# 12  of oscillators O1 02 ... 012 via switch contacts SC1, SC2 ... SC12 respectively; each oscillator is frequency-controlled by a reactance valve over the range 0.7-1.6 Mc/s. Oscillators 01 and 012 each have an additional, manual coarse frequency control. The oscillator outputs are fed via respective contacts OS1 ... OS12 to line ML. Oscillators 02 ... 012 can also be controlled in frequency by a signal fed through respective contacts S2 ... S12. Outputs from oscillators O1 . . . 011 are taken through respective contacts C1 . . . C11, which can be closed two at a time, to a mixer M1 whose difference-frequency output is applied to a frequency and phase discriminator FC the output D.C. voltage from which controls F# 12  via contacts S12. Frequency dividers (¸2) FD10 ... FD1 are added to provide for an oscillator output frequency F# N  of F# 1+  (F# 12 /2<SP>12.N</SP>) for oscillators 02 ... O10 and for oscillator O11 output frequency F# 11  of F# 1 -F# 1 /<SP>2</SP>. The frequency divider outputs are taken respectively through further contacts D1 . . . D10 to the comparator FC, and are taken to respective mixers MM1 ... MM10, the second inputs to which are taken from respective oscillators 01 ... 010; mixers MM1 ... MM9 each give a sum-frequency output and mixer MM10 a difference-frequency output. Frequency and phase comparators PD2 ... PD11 each have a first input from a respective oscillator 02 ... 011 and a second input from a mixer MM ... as shown; the phase comparators apply frequency frequency control to respective oscillators via contacts S2 ... S11. Target range is indicated by a frequency counter or frequency-responsive meter R, controlled by the frequency of oscillator 012. Mono-stable multivibrators Y 1  . . . Y 12  are connected in cascade and each, when triggered, actuates a corresponding relay Y 21  . . . Y 32 ; any one of the multivibrators may be the first of a cascade to be triggered, by a pulse applied by closing a switch Y 40 , by adjusting a selector Y 50 . It is necessary that the operator should know the range of a target, to be measured accurately by the apparatus, to within about Œ50%. Assume that the target is approximately at the full range of the apparatus: switch Y50 is set as shown, oscillator 01 is manually adjusted towards the lower end of its range, and oscillator 012 is set to a frequency equal to the velocity of light divided by twice the range estimate and multiplied by 2<SP>10</SP>. Before any relay is actuated the initial positions of the contacts in block Z is with contacts S2 ... S11 closed and all other contacts open. When the operator closes switch Y40, relay Y21 is actuated to close contacts SC1 and OS1, oscillator O1 frequency is then automatically adjusted to that " synchronous ranging frequency " (s.r.f.) which is nearest to the frequency to which it happens to be set, by the output from detector X12. Relay Y22 then makes so as to open contacts S2, SC1, OS1 and to close contacts OS2, SC2, C1, C2 and D1 ; oscillator 02 replaces oscillator 01 as the transmitter's modulating source and its frequency automatically adjusted by detector X12 output to that s.r.f. nearest to that frequency at which it was oscillating. Mixer M1 is simultaneously energized by the outputs of oscillators O1 and 02 and discriminator FC compares their difference-frequency with the frequency of divider FD1 to control the frequency of oscillator 012 until the discriminator inputs are of the same frequency and phase. Similar operations occur successively by relays Y24 ... Y32. Thus the s.r.f. to which oscillator 012 is ultimately set is unambiguous and accurate. If the approximate range of the target (a.r.t.) is > <SP>1</SP>/1.6 of the maximum range of the apparatus (m.r.), switch Y50 is put on position 1; if the a.r.t. lies between <SP>1</SP>/2x 1.6 and ¢Î0.7 of m.r., position 2 is chosen, and so on. If the a.r.t. is wrongly estimated for the position of switch Y50 chosen, oscillator 012 will endeavour to adjust its frequency outside its range of adjustment; an alarm may then be provided. Meter R may be calibrated with scales each covering a different one of the zones of range. To ensure faster operation electronic switching may be employed; or the switching operations may be controlled by the discriminators. Relay Y32 may have additional contacts to feed a pulse back to the armature of selector Y50 via an additional manual switch for use in excessive noise conditions.