Frequency shift modulator with circuitry for simple change-over between high and low channels

Claim 1

A modulator for data transmission by frequency shift modulation comprising a first operational amplifier and an associated capacitor connected so as to form an integrating circuit, a bistable circuit having trigger hysteresis connected to the output of the first operational amplifier for producing a two-level voltage, a first resistor having one end connected to the output of the bistable circuit and the other end coupled to an input of the first operational amplifier so as to produce a triangular voltage at the output of the first operational amplifier, first logic means for applying a voltage equal or complementary to said two-level voltage, depending upon the value of the data signal, to one end of a second resistor whose other end is joined with said other end of the first resistor, characterized in that for generating a first or a second frequency shift modulated signal having different central frequencies and being modulated with the same frequency shift the modulator comprises second logic means for applying a voltage equal or complementary to said two-level voltage, depending on whether the desired modulated signal is said first or said second frequency shift modulated signal, to one end of a third resistor whose other end is joined with the joined ends of the first and second resistors, and a common resistor connected between the joined ends of the first, second and third resistors and the input of the first operational amplifier.