Interference Suppression Performance of Several FM Receivers Using Feedforward

摘要

The feedforward signal-cancellation technique is based on subtractively combining the outputs of limiters and linear amplifiers having a common input. Used in an FM receiver, feedforward provides an attractively simple and effective method for suppressing interference to an FM signal from other co-channel or adjacent-channel signals which may be either weaker or stronger than the desired signal. The thesis explores theoretically and experimentally the potential performance and inherent limitations of practical FM receivers using feedforward. Design criteria are discussed for various interference conditions and the relative merits of several practical feedforward circuits are considered. A laboratory model FM receiver was built and tested with three different feedforward circuits, its performance being measured under a variety of interference conditions. Significant improvement in the stronger-signal capture performance of a mediocre FM demodulator was demonstrated. Sinusoidal modulation was recovered from FM signals between 0.05 and 0.9 times the amplitude of an interfering signal on the same channel, distortion ranging generally between 8 per cent and 30 per cent for various interference conditions. Completely intelligible speech modulation was also recovered from the weaker of two co-channel FM signals. (Author)