Patent GB-1505152-A (GB)
Title: Transponder system
IPC: G01S13/82
Filed: 1975-08-13
Abstract: 1505152 Transponders PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 13 Aug 1975 [16 Aug 1974] 33706/75 Headings H4L and H4P [Also in Division H2] The invention relates to a transponder system for railway signalling of the type comprising an interrogator station. Fig. 1, (e.g. on a train) and a passive responder station, Fig. 2, the interrogator being provided with a pulse generator 1 for a carrier frequency f 0 , and the responder having means for the conversion of f 0  to a harmonic, and an information source 30 coupled to a modulator for modulation of the harmonic to produce a return signal, the interrogator being further provided with a detector 44 for the return signal, the source 30 being coupled to the modulator via a switching circuit 25. According to the invention a code generator 33 is coupled via 25 to the modulator for the transmission of a continuous sequence of reply pulses in response to interrogation pulses, the interrogator being further provided with a decoder 51 coupled to 44, the decoder 51 being coupled to the pulse generator 1 for the insertion of a "start" signal in the interrogation signal after the detection of a predetermined number of modulated reply pulses, and the responder being further provided with a start signal detector 34 controlling a reply pulse modulated in accordance with the information derived from source 30. The carrier frequency f 0  is generated by an oscillator 2, and modulated by a switch 3 driven by a multivibrator 4. The responder has a circuit 10 tuned to f 0  and a circuit 11 tuned to 2f 0 , harmonics of f 0  being produced by a diode 12. Return pulses of frequency 2f 0 À are modulated by a circuit 20 including a varactor 21 shunted by a sweep generator 22, whereby the resonance frequency of 20 is swept over a band including 2f 0  so as to load circuit 11 and produce dips in the return pulses. Diode 12 also drives a unit 35 comprising a detector 36 for demodulation of the interrogator pulses to provide clock pulses, a smoothing circuit 38 which provides D.C. output power at a terminal V + for other units in the responder, and a threshold circuit 41 having its output connected to an AND gate 37. When the threshold voltage of 41 is exceeded clock pulses are supplied to a shift register 26 which holds binary information derived either from source 30, or from a code generator 33 providing a source of binary "1"'s to replace information displaced from the shift register at output 32. When an interrogator first approaches a responder the reply pulses are modulated with dips and detected by 44 as binary "0" signals. When the threshold of 41 is exceeded shift register 26 begins to produce an output, which eventually consists of a sequence of "1"'s from code generator 33, binary "1'"s resulting in the transmission of reply pulses without dips. A shift register 49, which may have for example eight stages, responds to the sequence of"1"'s by actuating decoder 50 to deliver a control signal to multivibrator 4 and produce the start signal. This may consist of either an extended pulse or a suppressed pulse, and it is detected by 34 the action of which is to control at 29 the parallel transfer of information from 30 to 26. Detector 34 may be a monostable having a time constant of such length that it is continuously set by the normal clock pulses, but switches over during a start signal of either type. The information transferred to shift register 26 is transmitted by the return signals and stored in a logic circuit 47 from where it may issue at output 52. Control bits may be incorporated into the return signal to check for errors, e.g. by dividing shift register 26 into a number of sub-registers with the control bits written into additional stages therebetween.