Multi Carrier Analysis of a Satellite Converter Output Amplifier

Nonlinearity behavior in microwave circuits generates intermodulation products that can interfere with the desired output signals. Linearity requirement of a microwave circuit is often specified by third-order intersect point (IP3) and 1 dB compression point (P1dB). These parameters are determined for an input of one or two signals whereas in reality microwave circuits often have a lot more than two input signals, and therefore it is desired to investigate the nonlinear behavior during a multi carrier case. In this thesis an investigation of the nonlinear behavior in an output amplifier when exposed to a multi carrier case has been performed. To perform the investigation a simplified simulation model was developed using the computer design software Advanced Design System. Measurements of a satellite converter output amplifier when subjected to multi carrier tests were conducted. This thesis also compares the simulation and measurement results for the multi carrier case. The figure of merit investigated, C/I, is determined as the relation between the carrier signals and the intermodulation products, and the relation between C/I, output power, IP3 and P1dB was studied for the multi carrier case. The results showed that C/I is similar for the all investigated multi carrier cases if compared at the same total output power, and C/I is therefore not strongly affected by the number of input signals. The results also showed that C/I is limited by IP3 at lower output powers, and by P1dB at higher output power. A breakpoint output power was found where C/I was affected by both IP3 and P1dB. The relation between the breakpoint output power, Pout, IP3 and P1dB for these measurements is given in the results. An important lesson from the thesis work is that the phases of the input signals are very significant for the output result of a multi carrier case. Thus when conducting a multi carrier simulation or test it is needed to use a suitable method to sum all contributions to the intermodulation product.Validerat; 20111012 (anonymous