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A new frequency-operated demodulator is described which does not respond to amplitude modulation. These results are achieved by making use of a controlled oscillator at the receiver which automatically synchronizes with the transmitter frequency. If desired, this method can be made to give a large response with extremely small amounts of frequency shift. On the other hand, it can also be used when the maximum frequency shift is many times as great as the signal band width. Other advantages are its linear response and its simplicity which requires the addition of only one tube to existing receivers. Tests show that a frequency-modulated signal and an amplitude-modulated signal may be applied simultaneously to the same carrier without noticeable interference at the receiver. Other applications of the synchronized oscillator are also described.