Abstract: An FSK modem transmission circuit which includes a modulator adapted to sctably produce pairs of low and high frequency carrier waves in any combination of up to five frequencies specified in international and U.S. standards. The modulator circuit includes a regenerative feedback loop which generates a triangular carrier wave at a frequency determined by the resistance of a resistor network having five different settings selected by switches therein controlled by the data signal and by logic control signals. The transmission circuit also includes a filter which modifies the waveform of the transmitted carrier wave to suppress odd harmonics thereof, thereby isolating the carrier wave from the modem receiver circuit which is coupled to the transmission circuit by a hybrid coupler.

Abstract: In an asynchronous transmission mode, the bit synchronization arrangement receives data signals in the form of binary pulse trains (at A). It includes an oscillator (3) whose output signal (at C) has a frequency F.sub.OS approximately equal to twice the binary rate of the pulse trains and a transition detector (1, 22) which supplies a calibrated voltage pulse (at B.sub.2) at each data signal transition. The oscillator supplies a triangular internal signal (at D) at the frequency F.sub.OS and a phase correction circuit (2) is provided to reduce the algebraic value of the slope of this signal during each calibrated pulse so as to produce a predetermined constant advancing or delaying phase shift according as the calibrated pulse occurs during a negative or a positive ramp of this signal.

Abstract: A modulator transmitting an analog data signal by variation of the frequency of an oscillator comprising an integrator (1, 2) and a bistable circuit with trigger hysteresis (8 to 13). The two-level voltage from the bistable circuit, taken to a terminal of a resistor network (6, 7, 30) connected to the input of the integrator, is formed from the potential O and the potential kU, which is lower than the supply voltage U, applied respectively to the inputs of two analog switches (12, 13) whose outputs are connected to the output of the bistable circuit. The data signal v.sub.e is applied to the input of a conversion circuit (17) arranged to supply voltages v.sub.e1 and v.sub.e2 varying linearly as a function of v.sub.e and symmetrically with respect to kU/2 and applied respectively to the inputs of two further analog switches (26, 27) whose outputs are connected to another terminal in the resistor network. The input and output signals of the inverter (9) of the bistable circuit are used to control the switches.

Abstract: A frequency demodulator including an "exclusive-OR" circuit receiving two square-wave signals having a frequency corresponding to a frequency-modulated signal, and a delay circuit for delaying one of the square-wave signals before application to the "exclusive-OR" circuit, by a delay varying versus frequency in such a way that the absolute value of the phase difference between the square-wave signals varies at most from 0.degree. to 180.degree. inversely with respect to changes in frequency. The output signal of the demodulator is supplied by a low-pass filter connected to the output of the "exclusive-OR" circuit.

Abstract: A test system is used to test a modem whose modulator is constituted by a multivibrator which during normal operation produces two frequencies f.sub.1, f.sub.2 in response to the data signal and which is connected to a transmission filter. In response to a test control signal, the multivibrator is modified to supply two test frequencies f'.sub.1 =nf.sub.1, f'.sub.2 =nf.sub.2, n being substantially equal to the ratio F.sub.o /f.sub.o, F.sub.o being the average value of the frequencies received by the modem during normal operation, f.sub.o being the average value of the frequencies f.sub.1, f.sub.2 ; at the same time the transmission filter is modified to transmit the two frequencies f'.sub.1, f'.sub.2 in response to the test control signal.

Abstract: The frequency shift modulator comprises an operational amplifier connected so as to form an integrating circuit and followed by a bistable circuit having trigger hysteresis for producing a two-level voltage which is applied to one end of a first resistor whose other end is coupled to an input of the operational amplifier. Logic means are provided for applying a voltage equal or complementary to the said two-level voltage, depending upon the level of the data signal, to one end of a second resistor whose other end is joined with the said other end of the first resistor. Further logic means are provided for applying a voltage equal or complementary to the said two-level voltage, depending on whether the desired modulated signal has either the one or the other central frequency, to one end of a third resistor whose other end is joined with the joined ends of the first and second resistors.