Track to train communication systems

Claim 1

A transponder circuit, for transmitting a coded message to an interrogator, comprising: a) an aerial for inductively coupling with the interrogator, the transponder circuit being energised through the inductive coupling and the transponder transmitting its coded message to said interrogator through the inductive coupling, b) a first capacitor connected in a circuit with said aerial whereby a tuned circuit is provided tuned to a first frequency, and c) a phase coherent frequency modulator comprising: (i) a second capacitor connectable into said tuned circuit whereby to retune said tuned circuit to a second lower frequency, the relative values of said first and second capacitors being such that said first frequency is an integer multiple of said second frequency, (ii) switching means arranged to connect the second capacitor periodically into and out of the tuned circuit, (iii) a data memory whose stored data is output in a predetermined timed sequence, when said transponder circuit is energised, to provide a sequence, of control signals to said switching means whereby to switch said second capacitor into and out of said tuned circuit according to a predetermined sequence whereby a coded message is output from said transponder circuit, each bit of which is made of a predetermined combination of cycles of said first and second frequencies, and (iv) a clock energised through said inductive coupling and whose clock pulses control the timing of the control signals output from said memory, whereby the switching between said first and second frequencies occurs with phase coherence and whereby the switching of the second capacitor into and out of said tuned circuit occurs at voltage maximum whereby energy is stored in said second capacitor when it is disconnected from the tuned circuit and is released when said second capacitor is reconnected to the tuned circuit.