High power waveform engineering

• Main Author: Sheikh, Aamir
• Published: Cardiff University 2010
• Subjects: 621.381
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.584991

For many years industry has considered RF PA design to be a "black art". This perception has been held due to the lack of availability of meaningful information for analysis and design. Due to the emergence of large signal waveform based measurements and increased understanding in the literature, it is now possible to characterise devices and correlate the information for enhanced PA design in terms of efficiency, linearity and/or reliability. This has been well documented and demonstrated using on-wafer devices but where this thesis work begins, little work had been done in expanding this capability to higher more meaningful power levels using packaged devices. This work has successfully addressed both of these limitations and extended visibility of time domain waveform data to higher power levels. Thus, allowing for the uncovering of world record efficiency levels of 77 % (4W output power) for Si LDMOS devices at S band frequencies using waveform engineering based procedures, in this case Class F. A feat previously only reported at L-band frequencies. Other waveform based designs such as inverse Class F and Doherty modes of operation are also successfully demonstrated in this thesis. In both of these cases, voltage related issues affecting reliability were uncovered that merit further consideration in the design process. Waveform engineering was made possible by applying de-embedding the measured current and voltage waveforms to the current generator plane. That is the plane at which the device is free from any device and package parasitics with the current and voltage waveforms seen to be in good agreement with those typically found in literature. These were successfully applied at high power levels (110W) previously not reported. To further demonstrate the relevance of waveform de-embedding, a non-linear charge conservative model based on industry standard modelling techniques was compared against time-domain measurements conducted in several classes of operation. This form of model verification is often overlooked and provides a unique insight into the model's accuracy highlighting new areas of improvement. In most cases the model was shown to be in good agreement with measured data, providing a high level of confidence in the application of waveform engineering principles within the CAD domain. Thus providing the PA designer the facility to apply waveform engineering on both the test bench and within the CAD domain.