Impulse noise in narrow-band fm receivers

THE RELATIVELY NARROW BAND FM communication receivers designed for use in mobile equipment are subject to disturbances caused by ignition systems in automobiles as well as to other disturbances of a similar transient nature. R-f signals are radiated from such ignition systems and these noise signals combine with the desired carrier to produce impulse-noise interference. When the disturbances are of a magnitude considerably greater than the desired information signal, and assuming that the receiver operates in accordance with the theoretical principles of frequency modulation, the receiver will respond only to the transient noise signal for the duration of the noise impulses. This results in receiver “blanking.” If the disturbing signal is of approximately the same amplitude as the desired signal, the two signals combine in the receiver to produce sudden changes in phase. These sudden phase variations result in large frequency excursions at the discriminator and cause shock excitation of the audio section of the receiver. Thus, when the interfering signal is either greater than or approximately equal to the amplitude of the desired signal, information will be lost. It is, therefore, desirable to design the frequency-selective elements in such a manner as to minimize this type of interference.