New Technique for Electronic Tuning of VHF Power Amplifiers

A technique is described for high-speed electronic tuning of an inductive device capable of handling VHF powers in excess of 100 watts CW. Electronic tuning is accomplished by means of a controlled dc bias applied to an inductor with a ferrite core. The orthogonal relationship of the applied dc magnetic bias and the absorption and dispersion of the ferrite is fundamental to circumventing problems that hitherto have limited the use of ferrite in magnetically tuned circuit,,; handling RF power. As opposed to conventional magnetic-bias tuning, the ferrite core is operated above saturation. In this mode, it is seen that the ferrite cores can be operated with high RF fields with no detectahle degradation of Q and negligible distortion products. Specific examples are presented of file application of this new capability in medium-power amplifiers operating over the frequency range of 70-300 MHz.