Patent GB-1290097-A (GB)
IPC: G01S13/74
Filed: 1970-03-24
Abstract: 1290097 Radio signalling UNISEARCH Ltd 24 March 1970 [2 April 1969] 14100/70 Heading H4L [Also in Division G4] In an electronic surveillance system, two signals are transmitted simultaneously which have different and widely spaced frequencies, and which when picked up by a passive label located within the near field region of the lower frequency signal are mixed to produce a reply signal which is distinctive in frequency from said transmitted signals. The reply signal is processed in equipment which is also responsive to at least one of the transmitted signals, and which verifies the presence of a label by reference thereto. As described, a main transmitter 1, Fig. 2, has a frequency of 915 MHz and emits pulses of length 10 micro-seconds and of repetition frequency 5000/sec. An auxiliary transmitter 2 has a frequency of 100 kHz and emits pulses of length 1 milli-second and of repetition frequency 93/sec. A label 3 comprises a stripline microwave aerial A, Fig. 3, which is operable at frequencies of 915 and 1830 MHz, and which has two sections separated by a thin dielectric film where they overlap, a semi-conductor diode B, capable of frequency doubling and frequency mixing, and a loop aerial C, operable at a frequency of 100 kHz. When the transmitter 2 is off, the signal from the label 3 comprises the 1830 MHz second ha monic, and when the transmitter 2 is on, the signal from the label 3 also contains 18300.1 MHz sidebands. Said signals are received by a receiver 4 which is tuned to 1830 MHz, and has a 300 kHz pass band. An AGC system 5 controls the gain of the receiver 4 and acts only on signals received while the main transmitter 1 is on and the auxiliary transmitter 2 is off, so that the 1830 MHz received signal is brought to a standard level, and propagation path losses are compensated. The output from the receiver 4 is processed in an analyser 6, during periods when the transmitter 2 is on, and responses are accepted as genuine when the sidebands are present in sufficient proportions. In a second embodiment, a main transmitter (15), Fig. 4 (not shown), operating at 915 MHz, an auxiliary transmitter (16), operating at 100 kHz, and a label (17), similar to the label shown in Fig. 3, are used but a receiver system (18) is tuned to receive the 915À1 MHz sideband generated by mixing the two transmitted signals in the label, and to reject 915 MHz signals. Synchronous detection is used, and direct connections from the transmitters to the receiver carry the necessary phase information. Time gating may be used to ensure that only responses within a pre-determined distance from the receiver are accepted. In a third embodiment (Fig. 5, not shown), in which main and auxiliary transmitters operating respectively at 915 MHz and 100 kHz are also used, a label 21, Fig. 6, comprises a single turn loop aerial 24, which receives microwave pulses from the main transmitter, a surface acoustic wave delay line 25, which produces after a time of approximately 500 nanoseconds an electro-acoustic echo of each transmitter pulse, which is re-radiated by the aerial 24, a modulation diode 26, and a multi-turn loop aerial 27, which receives from the auxiliary transmitter. The output from the label 21 comprises pulses of frequencies 914.9 MHz, 915 MHz and 915À1 MHz, the first and third resulting from intermodulation of the received 100 kHz and 915 MHz signals by the diode 26. All three pulses are delayed by 500 nano-seconds from the originating pulse from the main transmitter, and they are separated therefrom in a time gated receiver, and are processed.