Active device characterisation

The small-signal S-parameters of an active device (such as a FET or an amplifier etc) do not fully characterise the device. Power dependent S-parameter measurements are a simple way to characterise the non-linear properties of active devices. These measurements provide very useful knowledge about the device such as gain compression, output power levels and saturated power levels determined from the actual device. They allow the end-user to verify the modelling tools and to use the designed parts with greater confidence.

We have the capability to characterise 4 port devices to 70 GHz. Our capability includes device characterisation at fundamental and at harmonic frequencies. Using frequency extender heads, power controlled two-port device measurements to 750 GHz are possible.

Load-pull is used to characterise active RF devices under different load impedance conditions. NPL has load-pull capability to 30 GHz where the load impedance is set using either mechanical impedance tuners (passive load-pull) or signal injection (active load-pull). Load-pull measurements are used for model and design validation in amplifier development.

                                                                                           Schematic diagram of a passive load-pull system

Contours of power delivered to the load by an amplifier as a function of load impedance. The load impedance which optimises the delivered power is identified. The contours are plotted on a Smith chart representing the load impedance.

Test fixture for load-pull measurements of high-power FETs. The fixture includes a heat sink, 10:1 impedance transformers and a TRL calibration kit to allow the fixture to be characterised and subsequently de-embedded from the measurements

X-parameters (X-parameters is a registered trademark of Keysight Technologies) are a generalisation of scattering parameters (S‑parameters). They are a frequency domain black box behavioural model that can be used to describe the behaviour of nonlinear RF components such as amplifiers. They capture behaviour such as harmonic distortion and gain compression. They can be extracted from measurements of components made on a nonlinear vector network analyser (NVNA) and, once extracted, can be used to represent nonlinear blocks in a circuit simulator such as a harmonic balance simulator.

We have available an NVNA (nonlinear vector network analyser) operating to 30 GHz and would welcome the opportunity to work with interested parties on X-parameter measurements. We also have the capability to extract load-dependent X-parameters which are a generalisation of X-parameters to components such as transistors operating under highly mismatched conditions.

                                                                   NVNA set-up for measuring X-parameters

Publications

Stant L. T., Salter M. J., Ridler N. M., Williams D. F., Aaen P. H. ‘Propagating Measurement Uncertainty to Microwave Amplifier Nonlinear Behavioral Models’. IEEE Transactions on Microwave Theory and Techniques. February 2019, 67(2), pp. 815–821 

Knowing the measurement uncertainty allows us to specify a region or interval within which the true value of the quantity of interest should lie. We have offered S-parameter measurements to customers with stated uncertainties for several decades using NPL’s primary impedance microwave measurement system (PIMMS).

The Vector Network Analyser Dynamic Uncertainty Option (VNA-DUO) allows us to reliably evaluate the uncertainty in measurements of active devices operating in linear and non-linear modes of operation.

                                                 PNA-X network analyser running VNA-DUO to characterise an active device

Publications

D. Singh, M. Salter, J. Skinner, and N.M. Ridler, “Commissioning of a VNA dynamic uncertainty tool for microwave S-parameter measurements,” NPL Report. TQE 16, 2021. 

D. Singh, M. J. Salter, H. Votsi, and N. M Ridler, “Inter-laboratory comparison of S-parameter measurements with dynamic uncertainty evaluation,” 94th ARFTG Microwave Measurement Symposium (ARFTG), 26-29 January 2020, San Antonio, Texas, USA.

D. Singh, M. Salter, and N.M. Ridler, “Comparison of Vector Network Analyser (VNA) calibration techniques at microwave frequencies,” NPL Report. TQE 14, 2019.

R A Ginley, “Kicking the tires of the NIST microwave uncertainty framework, part 1,” 88th ARFTG Microwave Measurement Conference (ARFTG), Dec 2016

R. A. Ginley, “Kicking the tires of the NIST microwave uncertainty framework, Part 2,” 90th ARFTG Microwave Measurement Symposium, Dec. 2017.

The Nonlinear Microwave Measurement and Modelling Laboratories (n3m-labs) are dedicated to measurement and modelling of nonlinear RF and microwave technologies. There is one based within the Advanced Technology Institute at the University of Surrey and another in the Electrochemical and Electromagnetic Technologies (EET) division at the National Physical Laboratory in Teddington. More information on n3m-labs is available here.