Intelligence driven load-pull measurement strategies

The objective of this thesis is to provide improved load-pull measurement strategies based on an open-loop active load pull measurement system. A review of the evolution of non-linear measurement systems as well as behavioural model generation approaches has been presented. An intelligence driven active load-pull system has been presented in this thesis, based on deriving local PHD models to aid the prediction of the desired active signal in order to achieve a target reflection coefficient. The algorithm proved to be effective in reducing the number of iterations in an open-loop active load-pull system and thus improving the utilisation efficiency. A non-linear measurement approach suitable for wafer mapping and technology screening applications has also been presented as an application of this new algorithm. In this thesis, it has also been shown how the Cardiff Behavioural model is effective in its ability to interpolate or extrapolate non-linear measurement data and thereby improve the quality of measurement data and speed of measurement systems. This investigation was carried out in two stages; fundamental interpolation testing and harmonic interpolation and extrapolation testing

On the development and automation of a high-speed load-pull system based on Pxie modules

Recent RF applications and research require thousands of accurate measurements to be performed within a practical time. For instance, the global model extraction of a DUT requires thousands of accurate measurements, which would take a very long time when using the traditional RF measurement systems because they are relatively slow. Moreover, the inaccessible software that is used by the traditional systems has made them a vendor-defined system, where their application cannot be extended or amended. This is contrary to the need for a flexible RF system that can be extended and modified according to user preferences. Furthermore, the traditional load-pull measurement strategies are time-consuming thanks to the iteration process and the need for the user interaction. Therefore, developing a new high-speed measurement is essential. This work demonstrates a high-speed load-pull measurement system that maintains flexibility, accuracy, speed, and high dynamic range. The system’s architecture is based on PXIe modules, where the signal Thoalfukar Husseini detection is achieved through the use of vector signal analysers (VSA) that can operate over 50 MHz frequency bandwidth. The RF signal generation employs vector signal generators (VSG) using continuous wave (CW) mode generation. The system is calibratable over a 100 dB dynamic range and the measurement speed approaches 200 measurements/ sec at 10K samples/average. Due to the accessibility of the raw measured data and the customisable written software, statistical information has been employed to monitor the quality of the measurements and the status of the system. Moreover, an automated active load-pull measurement has been implemented on this measurement system. The automated process has been achieved by exploiting the load-based Cardiff behavioural model. This model is used to predict the required injected signals a21 to emulate a load impedance at the DUT reference plane, wherein the results show the ability of the model to achieve a load-target with an error less than -35 dB. The prediction of the DUT’s response by the model combined with customised software has allowed for an automated fundamental active load-pull process that requires minimum user interaction to automatically identify the optimum load conditions for the design-relevant parameters (e.g. gain, efficiency or output power over one or multi-power levels) within a few seconds. Two methods have been used to take the impact of the test-set on the generated signal into account: descriptive function and ix Thoalfukar Husseini simple look-up table. These two approaches have been implemented and verified. The results show that each model can achieve the power target with a residual error of less than 0.1 dB. The automation process has not only covered the definition of the optimum impedances over different power levels but has also identified, in a time efficient manner, the appropriate load-pull impedance space. This ensures that the model’s coefficients, which are required for predicting the DUT’s response b21 and efficiency, are accurately extracted. This approach significantly reduces the number of required measurements, and hence reduces the measurement time when compared to the traditional approach. It takes less than 42 sec to perform 1282 load-pull measurements, that define the appropriate design space (-3dB power contours) for 16 power levels while ensuring that the a- wave based Cardiff behavioural model is simultaneously and accurately extracted. For the sake of an efficient utilization of the measurement system and further reduction in the required number of measurements required to generate a global behavioural model that is compatible for CAD-tool design, a linear interpolation approach over the extracted coefficients was employed and verified. This approach has allowed x Thoalfukar Husseini further reduction in the number of measurements because there is no need to perform the load-pull measurement over a high dense grid of input drive power levels (a11), which is essential for the global model generation

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

Measurement techniques for the characterization of radio frequency gallium nitride devices and power amplifiers

The rapid growth of mobile telecommunications has fueled the development of the fifth generation (5G) of standards, aiming to achieve high data rates and low latency. These capabilities make use of new regions of spectrum, wider bandwidths and spectrally efficient modulations. The deployment of 5G relies on the development of radio-frequency (RF) technology with increased performance. The broadband operation at high-power and high-frequency conditions is particularly challenging for power amplifiers (PA) in transmission stages, which seek to concurrently maximize linearity and energy efficiency. The properties of Gallium Nitride (GaN) allow the realization of active devices with favorable characteristics in these applications. However, GaN high-electron mobility transistors (HEMTs) suffer from spurious effects such as trapping due to physical defects introduced during the HEMT growth process. Traps dynamically capture and release mobile charges depending on the applied voltages and temperature, negatively affecting the RF PA performance. This work focuses on the development of novel measurement techniques and setups to investigate trapping behavior of GaN HEMTs and PAs. At low-frequency (LF), charge dynamics is analyzed using pulsed current transient characterizations, identifying relevant time constants in state-of-the-art GaN technologies for 5G. Instead, at high-frequency, tailored methods and setups are used in order to measure trapping effects during the operation of HEMTs and PAs in RF modulated conditions. These RF characterizations emulate application-like regimes, possibly involving the control of the device’s output load termination. Therefore, an innovative wideband active load pull (WALP) setup is developed, using the acquisition capabilities of standard vector-network-analyzers. Moreover, the implications of performing error-vector-magnitude characterizations under wideband load pull conditions are studied. Finally, an efficient implementation of a modified-Volterra model for RF PAs is presented, making use of a custom vector-fitting algorithm to simplify the nonlinear memory operators and enable their realization in simulation environments