A threshold extension technique for frequency modulation systems



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The performance of an EM demodulator can be defined in terms of the threshold phenomenon which determines the minimum acceptable input SNR for the system. The presence of high-amplitude noise impulses ('click' noise) in the demodulator output is a primary factor contributing to the occurrence of threshold in an EM system. Certain distinguishing characteristics of click noise provide a basis for practical techniques which can be implemented at the output of an EM demodulator to improve its threshold performance. A threshold extension device has been developed which operates on the principle of click detection and elimination. Previous work in the area of EM demodulator improvement has concentrated on the design of specialized demodulation schemes which provide optimum performance for one set of channel parameters. However, EM systems use several channels having different characteristics in terms of required channel bandwidth, maximum frequency deviation, and range of modulation frequencies. The click-noise eliminator offers an advantage in flexibility since it is not a demodulator but rather a device which can be implemented at the output of any EM discriminator to provide improved system performance.