Power Amplifier Linearity Studies for SSB Transmissions

Intermodulation distortion measurements were made on traveling-wave tube amplifiers to determine the suitability of this tube type for use in single-sideband (SSB) frequency division multiplex transmitters. It was found that coupled cavity tubes exhibit better intermodulation distortion properties (by a factor of 5 or 6 db) than helix tubes. By employing a direct narrow-band feedback loop around a helix TWT, intermodulation distortion can be reduced by the amount of available loop gain. Both third-order distortion and noise-power ratio NPR measurements were made as a function of TWT output power. By using feedback techniques, it appears that a TWT ground transmitter could be made with sufficiently low distortion to satisfy CCIR requirements in the overall ground-spacecraft system without the use of compandors.