Frequency Step-Tuning Characteristics of Traveling-Wave Tube Regenerative Oscillators

A lack of compact, powerful, and efficient sources in the submillimeter-wave or terahertz (THz) frequency regime (~ 300-3000 GHz) has motivated the research on microfabricated vacuum electronic devices. Regenerative feedback traveling-wave tube (TWT) oscillators have been proposed and validated as good candidate sources of THz-regime radiation. Recently, a microfabricated folded-waveguide (FWG) TWT oscillator has demonstrated a peak output power of 50 mW at 656 GHz with an intrinsic circuit efficiency of 0.45%. By continuous variation of the beam voltage, the operating frequency was varied between 607 and 675 GHz. However, the oscillation frequency exhibited a step-tuning behavior, rather than a smooth variation with beam voltage. It is hypothesized that this step tuning results from a combination of the TWT gain characteristics and a phase self-consistency condition. To confirm this hypothesis, we theoretically model a 40-GHz FWG-TWT oscillator and experimentally study a 5-GHz helix-TWT oscillator. The FWG-TWT results exhibit an inclined step-tuning characteristic, similar to previously unreported observations on a 650-GHz THz-regime oscillator. Similar characteristics are also observed, along with multifrequency oscillations, in the helix-TWT experiments.