Harmonic gain and noise in a frequency-doubling gyro-amplifier

Nonlinear gain in a 34-GHz three-stage frequency-doubling gyro-traveling wave tube (gyro-TWT) has been experimentally studied. The device consists of a thermionic electron gun, TE₀₁→TE₀₂ fundamental gyro-TWT input section, second harmonic TE₀₃ intermediate buncher section, and a second harmonic TE₀₂→TE₀₄ complex output circuit. Nonlinear bunching in the electron orbital phase generates harmonics of the input signal in the beam current, which excite the subsequent circuits at the second harmonic frequency. Since the gain is nonlinear, noise or applied sideband signals intermodulate with the carrier generating high-order products in the output. Therefore, it has been suggested that the noise figure of these devices may be unreasonably high. In this study, the complex harmonic transfer characteristics were experimentally measured and compared with calculations based on the assumption that the gyro-amplifier gain can be described, in the narrowband sense, as a classical frequency-doubling circuit. The results show that narrowband intermodulation gain is 6 dB higher than the carrier as predicted in the small signal limit, but as the device reaches saturation the nonlinear products become suppressed with respect to the carrier. Tests on the broadband gain characteristics show that output noise consists of second harmonic shot noise spontaneously excited in the output circuits along with the products of the intermodulation between external noise and the carrier. Good agreement between the experimental results and the calculations is demonstrated.