A Frequency-Dividing Locked-in Oscillator Frequency-Modulation Receiver

A new type of frequency-modulation receiving system is described in which a continuously operating local oscillator is frequency-modulated by the received signal. In an embodiment of the system which is described, the oscillator is locked in with the received signal at one fifth the intermediate frequency. With this 5:1 relationship between the intermediate frequency and the oscillator frequency, an equivalent reduction in the frequency variations of the local oscillator is obtained. Received signal-frequency variations of ±75 kilocycles are reproduced as ±15-kilocycle variations in the oscillator frequency. The frequency-modulated signal derived from the oscillator is applied to a discriminator which is designed for this reduced range of frequencies. The oscillator is designed to lock in only with frequency variations which occur within the desired-signal channel. The oscillator is, therefore, prevented from following the frequency variations of a signal on an adjacent channel. A substantial improvement in selectivity is thus obtained. The voltage required to lock in the oscillator with a weak signal is approximately one twentieth of the voltage applied to the discriminator. Since this voltage gain is obtained at a different and lower frequency than the intermediate frequency, the stability of the receiver from the standpoint of over-all feedback is materially improved. Other performance advantages and the factors affecting the operation of the sytem are discussed.