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An amplitude-modulated (AM) ultrasonic range finder using an online parameter estimation procedure is presented, which uses the sliding discrete Fourier transform (SDFT) algorithm for extracting the sinusoidal envelope from the received reference and ultrasonic signals. The received ultrasonic envelope contains an additive noise, which resembles another sine wave whose frequency is very close to that of the envelope. This gives rise to a low-frequency beat in the phase shift between the transmitted and received envelopes. Consequently, the estimated phase shows an equivalent phase jitter. The desired sinusoidal envelope signal cannot easily be separated from the noise signal, even by sharply tuned SDFT filters with phase-locked loops (PLLs). A parameter estimation procedure has been applied to remove these interharmonic signals with the help of comb filters. The PLL was strengthened by a cosine lookup table (LUT). By locating the envelope frequency far away from the interharmonic noise frequency, the convergence time could greatly be reduced. Simulation studies were conducted in the Matlab-Simulink-DSP builder environment, and ideas were implemented in a Cyclone-II field-programmable gate array (FPGA)-based range finder fabricated in the laboratory. The test results of the AM ultrasonic range finder are presented to show its performance for static and slowly moving objects.