Effects of Load Impedances at Third Order Intermodulation Tones

This paper reports on new multitone active loadpull measurements at the specific frequencies of third order intermodulation distortions (IMD3). The achieved separation between the effects of fundamental and IMD3 loadpull quantify clearly the potential of incorporating IMD3 load impedances into low distortion high efficiency amplifier design. The results show an increase in drain efficiency (DE) of 11 points and 13dB drop in IMD3 levels by varying the IMD3 impedances only. The results demonstrate the same IMD3 optimum impedances for achieving both maximum DE and linearity yet significantly different from impedances for optimum DE at fundamental frequencies, hence strongly suggesting the separate tuning of IMD3 impedances for improving amplifier's DE and linearity. This is the first investigation of loadpull at specific IMD3 tones covering the entire smith chart at realistic frequency spacing and compression levels.