DESIGN OF A BFSK COMMUNICATION SYSTEM

Abstract
Design of Binary frequency shift keying modulator and demodulator is discussed here with an emphasis on non-coherent detection as this is the main objective of the project. The modulator consists of two oscillators producing sinusoids of 10kHz and 15 kHz sampled together with an information data of 1kHz. For demonstration purposes the data input is a random binary sequence obtained from a Pseudo noise sequence generator which has also been included in the design. The chosen frequencies coincide with the requirements of non-coherent detection. i.e. the tone interval has to be an integral multiple of the sampling rate. The two identical sampling circuits consist of a P-channel JFET and act as a switches that amplitude modulate the two sinusoids with sympathy to the sequential binary input at the gates. One sampler takes the normal data input while the other takes an inverted binary data. A summing op-amp is used to combine these two frequencies. The demodulator constitutes two bandpass filters matched to the two carrier signals, an envelop detector and a comparator to reproduce the information data.
Tests and results are achieved by practically building the circuit on a breadboard in the laboratory. Instruments used are the oscilloscope, Dual, 5V and ±15 power supply and a signal generator. Due to the use of standard resistor values 10.41kHz and 15.15. KHz are obtained from the oscillators. This doesn’t degrade the performance at the detector as this scheme doesn’t have fixed frequency constraints. The modulated signal is successfully achieved. Demodulation is accomplished by band pass filtering and envelop detection and at the comparator output an open collector resistor tied to 5V produces a digital signal output equivalent to the binary input signal. The difference is a small delay owed to the time constant in the envelop detector

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