In the last few years, the field of microwave testing has been evolving rapidly with the development and introduction of digital techniques and microprocessor based instruments, and reaching higher and higher frequencies. Nevertheless, the basic underlying concepts, such as frequency synthesis, network analysis and calibration, and spectrum analysis, still constrain even the more modern equipment.

In recent years, microwave instrumentation has had to meet new testing requirements, from 3G and now LTE wireless networks, for millimeter wave and THz applications. Thus instrumentation and measurement techniques have evolved from traditional instruments, such as vector network analyzers (VNAs), to increasingly more complex multifunction platforms, managing time and frequency domains in a unified, extensive approach. We can identify two main directions of evolution:

• linear measurements, essentially S-parameter techniques;

• nonlinear measurements, for high power and nonlinear device characterization.

S-parameter measurements have been moving towards the multiport and millimeterwave fields. The first to characterize multi-channel transmission structures such as digital buses, and the latter for space or short-range radio communication or security scanner applications. New calibrations and instrument architectures have been introduced to improve accuracy, versatility and speed.

Nonlinear applications have also evolved. Traditional high power transistor characterization by load-pull techniques now also typically includes time domain waveform measurements under nonlinear conditions. These techniques can nowadays also handle the broadband signals used in most communication links, or pulsed signals. Moreover, even nonlinear measurements had to evolve to multiport, with differential and common mode impedance tuning, due to the spreading of amplifiers and devices exploiting differential configuration.