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An ultrasonic range finder for mobile robots has been proposed which uses the sliding discrete Fourier transform (SDFT) for extracting the low frequency sinusoidal envelope of a modulated ultrasonic signal. The continuous wave method uses the phase shift between the envelopes of infrared and ultrasonic carriers modulated by identical low frequency sine waves, for range measurement. The infrared sensor receives the reference waveform almost instantaneously, while the envelope of the modulated ultrasonic signal undergoes a phase-shift proportional to the distance traversed. Any frequency-drift in the envelope, which may occur at the sending or the receiving end, is tracked and compensated by the integrated phase locked loop (PLL). The fundamental components corresponding to the envelopes of the received reference (IR) as well as the ultrasonic signals are extracted using the SDFT procedure, in real time. The phase shift is determined by the Park transform procedure. Extensive simulation studies have been made, in the MATLAB DSP Builder environment so as to implement the scheme into a single Cyclone-II FPGA chip.