Microwave ferrite modulators for high signal frequencies

This paper discusses the design problems of microwave Faraday modulators for use with signal frequencies at the lower end of the megacycle spectrum. The frequency limitation caused by skin effects in the guide wall is overcome by using the waveguide as a modulating helix. It is shown that the other important factor is ferromagnetic resonance phenomenon in the ferrite at the signal frequencies. Means are suggested for avoiding this limitation by using ferrite compositions which give reasonable performance at both the carrier and signal frequencies. Details of the modulation conversion efficiency of two experimental modulators are given, and these show that a first order sideband output 15-20db down on the input power of an X-band carrier can be realized.