Developing a multi-tone load-pull system for the direct extraction of Cardiff behavioural model coefficients

The main objective of this thesis is to develop and utilize a high-speed measurement system based on PXIe modules for multi-tone measurements. This thesis addresses challenges that have been tackled during developing an accurate LabVIEW software to measure the multi-tone signal. Having solved these problems, the system's functionality was demonstrated by using it to extract some useful data such as stability and gain information for RF designers in, ‘real time’ during active load-pull measurements. In the final part of the thesis, the system was used to aid the development of the Cardiff behavioural model. One of the main challenges in the development of the Cardiff behavioural model is to correctly select the required mixing terms to have an accurate model. Initial work has been focused on determining the correct, phase polynomial coefficients of the Cardiff Model. The first technique presented in this thesis utilises two-tone measurements and the Fast Fourier Transform (FFT) to observe the mixing order from the resultant intermodulation (IMD) products, which are directly associated with the Cardiff Model coefficients. Employing the IFFT, the selected tones have been transferred to the time-domain travelling-wave. This allowed for b-wave analysis and load modulation coverage to be seen on the Smith Chart. The resultant load-modulation from the two-tone measurements has been used as a target for CW impedance measurements to verify whether the identification holds for the CW domain. The result shows there is an VII offset between the CW measurements and the two-tone measurements for larger annuli as th

Cardiff behavioural model analysis using a two-tone stimulus

This paper presents a new technique for identifying the mixing structure, model coefficients and therefore model order of the Cardiff behavioral model for phase related nonlinearities. The technique employs a two-tone measurements approach and the Fast Fourier Transform (FFT) to be able to observe the mixing structure above the noise floor of the measurement system. Spectral tone visibility explicitly requires model coefficient inclusion for accurate (NMSE <; -40dB) data fitting, which is verified by comparing model fitting of full and truncated model formulations. The identified maximum phase model order from two-tone measurements, for annuli on the Smith Chart, is shown to be accurate for Continuous Wave (CW) measurements

On the effective modeling of the test-set non-linearity

This paper investigates and compares the use of nonlinear test-set models for an effective and accurate operation of active load-pull systems. The results demonstrate a simple-to-implement and yet robust technique to align the generator and receiver reference plane with a minimum set of required measurements. With only 14 measurements a high agreement between target and measured load points was achieved with an average error less than 0.1 dB over a 70dB dynamic range. An increase in modeling complexity has therefore yielded no improvement. To compare the results, a behavioral model was employed, and it is shown that a high order of model complexity is required to achieve the same level of accuracy. The presented work provides, for the first time, a practical and effective method for the modeling of test-set nonlinearities, hence allowing a cost-effective implementation of active load-pull systems that operate power amplifiers within a gain compression region

Global behavioural model generation using coefficients interpolation

This paper demonstrates a novel approach to reduce the density of load-pull measurements that are required to populate a high-density Cardiff behavioural model coefficient look-up table, versus input drive signal, covering a large dynamic range. The presented approach investigates the use of coefficient interpolation to achieve this objective using a recently published re-formulation of the Cardiff behavioural model. This approach provides global interpolation functions for the model coefficients with respect to input drive level (ja11j). It is shown that this approach can provide for accurate interpolation of load-pull behaviour over a 2-4 dB power range. This knowledge can be used to reduce the number of measurements necessary, hence the time duration required, to populate the Cardiff behavioural model look-up table, without compromising accuracy when used in CAD simulations. The techniques are demonstrated on a 10W packaged Cree HFE

Behavioural model extraction using novel multitone active load-pull

This paper presents a new measurement and data analysis approach for both identifying the required Cardiff behavioural model complexity and directly extracting the associated model coefficients, K p,h,m,n . The technique developed utilizes a multi-tone measurements approach. Load-pull measurements are performed using an engineered multi-tone active load-pull excitation, A 21 (t), that is chosen for its ability to identify the required model complexity. Fast Fourier Transforming (FFT) the device response, B 21 (t), allows the respective model mixing order contributions to be directly observed above the noise floor of the measurement system. Formulating the Cardiff behavioural model in the frequency domain, with this selected multi-tone stimulus, also allows for the first time the direct extraction of the model coefficients

Automating the accurate extraction and verification of the Cardiff Model via the direct measurement of load-pull power contours

The CAD design of Power Amplifiers requires an accurate non-linear modelling solution. Generally, this is provided by state function (1-V, Q-V) model formulations. These typically require time consuming measurement procedures for model extraction and verification. Look-up table a-wave based behavioral models, i.e. the Cardiff Model, extracted directly from measurement data provide for a robust alternative, addressing both simulation accuracy and model extraction time. The challenge is identifying, in a time efficient manner, the appropriate load-pull impedance space, that ensures the model coefficients are accurately extracted. This paper outlines an automated approach addressing this requirement, that exploits the novel features of emerging high-speed load-pull measurement systems to identify and then measure directly load-pull power contours. The automated approach reduces significantly the number of required measurements, hence the measurement time, compared with the traditional approach while also ensuring an accurate Cardiff Model is extracted. The approach is demonstrated on a 10W packaged Cree HFET