Patent GB-1171751-A (GB)
Title: Secure Signalling Systems.
IPC: H04K1/02
Filed: 1966-11-21
Abstract: 1,171,751. Radio signalling; pulse radar. NORTH AMERICAN AVIATION Inc. 21 Nov., 1966 [1 Dec., 1965], No. 52048/66. Headings H4D and H4L. In a communications system with provision for secrecy, two carrier waves of different frequencies are produced and are both modulated with the information to be transmitted. The frequencies of the carrier waves are varied according to a programme, with the frequency difference between them maintained at a predetermined value. For reception, non-linear detection means responsive to both modulated carriers produces an output from which components centred upon their difference frequency are selected by means of a bandpass filter, and from which the information may be recovered by further detection, As a further secrecy measure, the frequency difference between the carriers and the centre frequency of the bandpass filter may be varied together. As described, power from an R.F. oscillator 10, Fig. 1, at a frequency f(t) =f 1  which is varied in a predetermined manner is fed via a unit 11 to one input of a mixer 12 which has a second input at a frequency #f from a source 13. The output from the mixer 12 at a frequency f2=f 1 +#f is combined at 17 with the other output from unit 11 at a frequency f 1  and both carriers are modulated with information at 15. A gain/phase compensation unit 19 enables the power in each carrier to be made the same. For reception, a broad-band R.F. amplifier 23 feeds a square-law detector 21, from which output centred on the frequency #f is selected by a bandpass filter 23 and detected by known means. In another embodiment, Fig. 2 (not shown), the whole output at a frequency f(t) of an R.F. oscillator (10) is fed to one input of a balanced modulator (25), and the output of a source (13) at a frequency #f/2 is fed to its second input. The output of the balanced modulator (25) comprises two equal parts at frequencies f 1 =f(t) - #f/2 and f 2 =f(t) +#f/2 respectively, and these are modulated together in a modulator amplifier (15). In an application for IFF (Identification, Friend or Foe) in co-operation with a radar search system, a microwave coupler 32, Fig. 4, with an output to a radar aerial 31 has two inputs, from radar equipment 30 and from interrogation equipment, respectively. The interrogation equipment comprises a dual carrier frequency generator 42, a modulator 15 and an interrogation coder 35, and it is connected to the coupler 32 through a circulator 43. Reception of transponder coded dual frequency signals in reply to an interrogation is by an omni-directional aerial 44, a square law detector 21, a bandpass filter 24, and a decoder 38. A display is given on the indicator 33 associated with the radar system, for example, as a bar (50), Fig. 5 (not shown), adjacent the trace (48) of the interrogated target. Incoming dual frequency interrogation signals are also received in the units 44, 21, 24 and are decoded in a decoder 36. Replies to interrogation of the radar system are made by a transponder coder 37 operating a single frequency transponder 39 coupled to an omni-directional aerial 40. Single frequency incoming interrogations and single frequency replies to interrogation are received by the aerial 40 and the transponder 39, and are respectively decoded in the decoders 36, 38.