Advanced Time Domain Loadpull Technique for Characterization of Microwave Power Transistors
Verspecht, J¹; Teyssier, J-P²; De Groote, F^3
1Jan Verspecht b.v.b.a.; ²XLIM - University of Limoges; ³Verspecht-Teyssier-DeGroote s.a.s.

Introduction
Advanced measurement technologies for characterizing power transistors are introduced. The basic idea is to upgrade existing loadpull set-ups such that they are able to measure the time domain voltage and current waveforms at both transistor terminals. The upgrade is based on the combination of three technologies: low-insertion loss loop couplers ("wave-probes"), broadband microwave receivers and advanced calibration software.

Description of the novel setup
The main difference with classic loadpull techniques is that the RF signals are sensed between the device-under-test and the tuners, whereas in a classic loadpull setup the tuner is placed in between the device-under-test and the RF signal sensors. The new sensing method is realized by using a low-insertion loss loop coupler structure, also called a "wave probe". The outputs of the wave probe, the sensed incident and reflected traveling voltage waves, are connected to a broadband microwave receiver, which measures the phase and amplitude of the fundamental as well as of all significant harmonics.

The wave probe is actually a loop coupler structure with a tiny loop (significantly smaller than a quarter wavelength). The loop is exposed to the electro-magnetic fields of the RF signals as they travel through the waveguide structure. The loop introduces virtually no insertion loss, yet has a directivity that is sufficient for all loadpull applications.

The broadband receiver is based on a 4-channel sampling frequency convertor. The basic idea is to sample the RF signal at a rate that is slightly offset from a subharmonic of the fundamental frequency. As a result the intermediate frequency (IF) output of the sampler contains low frequency copies of the fundamental as well as the harmonics. The sampling process preserves the amplitude as well as the phase relationship between all of the spectral components. The IF output signals are digitized by standard analog-to-digital convertors with a typical bandwidth of 25 MHz.

The advanced calibration procedure is based on a classic VNA calibration procedure that is extended with an amplitude calibration, based on a power meter, and an harmonic phase calibration step, based on a pulse generator, a so called "harmonic phase reference generator".

Advantages of the novel method
The modern setup has many advantages when compared with the classic approach. Probably the most significant advantage is that the RF signals are sensed between the tuner and the device-under-test. As a result the measured RF signals completely determine the RF signals at the device-under-test terminals, it is not necessary to know the S-parameters of the tuner. In fact the information on the impedances represented by the tuner can be derived from the RF signal measurements. As such the new setup no longer requires any a priori characterization of the tuners. A second advantage is that the setup allows to determine the amplitudes and the phases of the fundamental as well as all significant harmonics of the RF signal. As such the data can easily be transformed into the time domain. Converting the traveling voltage waves in a current/voltage representation and plotting the time domain drain current versus the time domain drain voltage results in a representation of the so-called dynamic loadline, a popular tool for amplifier design.

Conclusions
The authors firmly believe that more advanced time domain loadpull setups like the ones described in this paper will become more readily available to a broad number of microwave engineers in the near future.