Frequency and Phase Locking of Magnetrons Using Varactor Diodes

Magnetrons are compact, inexpensive, and highly efficient sources of RF power used in many industrial and commercial applications. For most of these applications, the requirement is for RF power without regard to precise frequency or phase control, and noise riding on the RF signal is not important. For many accelerator, defense, and communications applications, however, these characteristics prevent use of magnetrons. It is possible to control frequency, phase, and noise by locking the output to a reference RF signal directed into the magnetron output. This requires a circulator to protect the RF source providing the locking signal. Unfortunately, circulators are large, often expensive component that prevents installation in compact or mobile applications. This program proposes to use varactor diodes to frequency and phase lock the magnetron RF power. The program will investigate two approaches. The first approach is to insert reactive components into the magnetron resonant structure to convert the magnetron into a voltage-controlled oscillator. The fast tuning speed of the varactor diode will provide rapid frequency tuning offer a significant bandwidth. The requirement is to use a varactor that can tolerate the high RF frequency and power loading. This approach incorporates simple hardware with high loop bandwidth and lower cost than previous investigations. No RF power is required to lock the magnetron, and no circulator is required. The second approach incorporates varactor diodes into a cavity coupled to the external waveguide that generates a controlled reflection toward the magnetron. The advantage of this approach is that no modifications are required to the magnetron with the mechanical and electrical issues that entails. The Phase I program will simulate performance in an S-Band magnetron currently in production. The program will investigate mechanical issues associated with integrating the diode into the RF circuit to provide the required performance, and perform experiments to test the external cavity approach. If the results demonstrate feasiblilty, it will be proposed to build and test a varactor controlled, S-Band magnetron producing more than 5 MW CW in the Phase II program.