3 for the time being it should only be noted that each back modulation stage 10 20 comprises at least one electronic switch at least one capacitor and at least one resistor to modify the load of oscillating circuit 2 and enable the corresponding detection on the terminal side fig2 shows a first embodiment of a so called back modulation stage according to the present invention such a stage comprises in parallel between terminals 3 and 6 at least one capacitor c 3 and one resistor r 3 both switchable capacitor c 3 and resistor r 3 are made switchable by being for example each series connected with a switch kc 3 kr 3 individually controllable by the processor of circuit 8 for example switches kc 3 and kr 3 are formed of mos transistors according to the embodiment of fig2 stage 10 comprises a similar structure between terminals 4 and 6 thus a capacitor c 4 and a resistor r 4 each in series with a switch kc 4 respectively kr 4 are connected in parallel between terminals 4 and 6 the different modulation elements are individually controllable to optimize the reception of data by the terminal according to cases it may be.