In this paper a method for increasing the bandwidth of a test-bed for dynamic characterization of power amplifiers is described. The technique is readily implemented using commercially available instruments which makes it suitable for e.g. production testing The bandwidth extension technique is combined with coherent averaging of the measurements in order to simultaneously increase the bandwidth and dynamic range of the test-bed. In addition the errors in the obtained wideband signal are estimated. The method is evaluated experimentally on a basestation power amplifier for the 3G WCDAM system.