Design of TWTs with small multi-carrier distortion characteristics

In multi-carrier operation, TWTs exhibit an intermodulation distortion power of typically 10 to 12 dB below the carrier at saturation. The distortion is commonly reduced by operating the tube below saturation, but at the expense of tube efficiency. A study was conducted on optimizing the design of G-band TWTs capable of amplifying between 4 and 16 equal-amplitude signals with a carrier-to-IM distortion power ratio of at least 20 dB, and demonstrating an overall efficiency of 25 percent minimum. The approach was to operate the tubes backed off from saturation for low distortion, and utilize a four-stage depressed collector to achieve the efficiency. Detailed designs satisfying the IM requirements were derived for a 1 kW PPM-focused tube and a 10 kW solenoid-focused tube operating in the band 4.4 to 5.0 GHz. The coupled-cavity circuits were operated closer to beam-RF wave synchronism than in standard designs. The analysis consisted of deriving the TWT transfer functions (output power and phase versus input power), computing the multi-carrier IM levels from the transfer characteristics, and establishing the collector design and performance from trajectory calculations and spent beam energy distributions.