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A new type of FM demodulator, the Frequency Locked Loop (FLL) is presented. The concept employed in this class of circuits is shown to be applicable in two forms. The first form is realized at the carrier, or IF frequency, to provide an FM detector that has the properties of not requiring a limiter preceding it and also of having a threshold superior to a normal FM discriminator limiter combination. The second form of the FLL is realized at baseband. This form may be added to any existing FM receiver to provide threshold extension properties to the original receiver. In addition to discussing both versions of this new circuit arguments as to the click suppression and threshold extension properties of the circuit are presented. Detailed experimental comparisons of the new circuit with a conventional FM discriminator are made. A comparison is also drawn between the theoretically possible threshold extension of an FLL and that actually obtained with a baseband version of the FLL.