Reduction of threshold noise in fm receivers operating in quasisynchronous field

The threshold effect is a phenomenon experienced in the reception of frequency modulated (FM) signals. It is found to be particularly troublesome in FM quasisynchronous (QS) mobile radio schemes, in the parts of the coverage area where carrier cancellation occurs due to carrier components interaction. This results in sharp noise bursts punctuating the receiver output and a consequent degradation in the speech quality. The paper describes a technique which was implemented as a receiver modification in the form of an add-on unit for the purpose of minimising the threshold noise effects with satisfactory results. Measurements conducted on a commercial receiver incorporating this modification revealed that an improvement of 12 dB in the output S/N was achieved.