Transmitter Power Reduction with Frequency Tracking FM Receivers

In analog communication studies much attention has recently been given to the apparent advantages of FM transmission with receivers employing feedback frequency tracking. Although theoretical investigation has indicated some carrier power reduction can be obtained over standard FM, it has been difficult to estimate the operating capabilities of the system. In addition, little has been published concerning the best parameter values for the construction of a typical feedback receiver. This paper is devoted to a study of the feedback FM receiver which maximizes power savings over standard FM for a given desired output SNR. System parameters such as transmitter modulation index, loop gain IF bandwidth, and baseband filter bandwidth are specified corresponding to this optimum design. Particular attention is given to the output SNR in the range from 15 db to 50 db where most practical analog systems usually operate. The paper deals only with the feedback loop containing two stages of filtering; design procedure is also given when added constraints are imposed in terms of loop transient response. The possibility of using phase-lock loops as the discriminator within the FM feedback loop is also investigated. Some experimental results are quoted which tend to verify the design techniques.