Pulsed multi-tone measurements for time domain load pull characterizations of power transistors

Real-time active load-pull (RTALP) characterization of power transistors with a large signal network analyzer (LSNA) has the benefit of greatly accelerating the design of power amplifiers. However RTALP has been so far limited to devices operating under CW excitations and constant biasing and was therefore mostly applicable to transistors with small memory effects. In TDMA communication or radar systems, amplifiers are excited by pulsed or modulated RF signals and operate under pulsed biasing. For such applications the measurement of modulated/pulsed RF signals under pulsed biasing/RF conditions is required for a more realistic understanding of the behavior of transistors with memory effects. This paper demonstrates that under appropriate conditions it is possible to effectively combine these two excitation modes while performing large-signal measurement by sub-sampling down-conversion with a LSNA. In this demonstration, the acquisition of CW two-tone and three-tone excitations is performed using multiple-recording with pulse duty rate of 1.5% and 0.15%. The pulse-recorded measurements for the three-tone excitation are found to be consistent with CW measurements (100% duty rate). This preliminary work indicates that either intermodulation or real-time active load-pull measurements under pulsed operation can be readily realized for RF samplers with sufficiently short settling time while operating under pulsed RF modulation.