Unwanted Emissions of Multi-Carrier Transmitter in Spurious Domain

We analyzed theoretically and experimentally the spurious emissions that a multicarrier transmitter used for satellite communications produces in the spurious domain defined in ITU WRC-03. The mechanism that generates spurious emissions in multichannel or multi-carrier transmitters/transponders—where several carriers may be transmitted simultaneously from a final output amplifier—is particularly complex. We paid particular attention to the spurious emissions generated due to the nonlinearity of a high-power amplifier (HPA) because the HPA is a dominant component in generating spurious emissions. It is particularly desirable to determine the emission level and conditions under which an HPA system is both efficient and meets recommended spurious emission levels. Therefore, we investigated the theoretical and actual characteristics of a multi-carrier transmitter. We found that the spurious emission levels are almost the same when the number of carriers is beyond eight and, therefore, we can estimate the worst spurious emission level from the eight-carrier result. Furthermore, we also studied its sensitivity to the nonlinearity and its dependence on the operating point of HPA, and the signal bandwidth for the purpose of system design. We developed a simple and cost effective evaluation method that uses one un-modulated (CW) measurement and a simple calculation to determine whether a system is compatible with the recommendations without performing complex multi-carrier tests. The analysis and our proposed evaluation method will help make possible the construction of powerefficient satellite multi-carrier systems.