Improvement of the signal/noise ratio of AM systems in the microwave region by means of a degenerative parametric preamplifier

The investigation is concerned with a nonreciprocal iterative network of two parametric mixers. The network makes it possible to improve the signal/noise ratio of amplitude-modulated signals by 3 dB. The approach employed for the improvement of the signal/noise ratio utilizes the different statistical characteristics of modulation and noise side bands. A pump frequency which is exactly twice as high as the carrier frequency of the AM signal is selected and a certain defined phase relation is established between pump and carrier (phase coherence). The term 'degenerative' is used for the considered type of operation of a parametric amplifier. Particular attention is given to a study of the mechanism involved in the improvement of the signal/noise ratio of the amplitude-modulated signals. The possibility of an implementation of degenerative mixer network amplifiers in the microwave region is demonstrated with the aid of an experimental circuit for operation at 4 GHz.