Envelope emulation

Abstract

Researchers from the UK present a novel characterisation system that is capable of emulating a complete envelope tracking power amplifier (ET-PA) environment around a microwave power transistor. The system achieves this by using modulated impedance synthesis and a dynamic supply voltage generation, as well as industry-standard modulated signals and digital pre-distortion. The authors believe that the system will be of use to designers of ET-PAs, allowing more complete investigation of the performance of ET-PAs in realistic modulated and load impedance conditions. As communication systems evolve to transmit ever-larger volumes of data, including the ongoing roll-out of 5G worldwide, we will need to develop more advanced modulation schemes to satisfy the rampant growth and to improve spectral efficiency. Whilst new modulation schemes offer higher data throughput levels, they are characterised by high peak-to-average power ratios (PAPRs). Power amplifiers (PA) are the major power consumer in mobile communication systems, and thus several power amplifier architectures have been proposed to improve average PA efficiency, including the Doherty PA and envelope tracking (ET). Authors (right to left) Sattam Alsahali, Alexander Alex,Dr. Jonathan Lees and Dr. Peng Chen. Set-up of the present work. Average PAE for 28 V fixed bias (blue solid contours) and for ET (green dotted contours). ET works by rapidly changing the supply voltage of the PA in response to the input signal, providing low supply voltage for low powers and vice versa. Whilst ET presents itself as a strong candidate for a wide range of applications due to its potential to support both multimode and multiband operations, the design of the ET-PA presents an array of challenges relating to optimisation of and interaction between involved sub-systems, in particular the radiofrequency (RF) potential of the ET-PA. Characterising the performance of a radiofrequency power amplifier (RFPA) and optimising its performance as early as possible within the ET environment is highly desirable. Load-pull, the colloquial term for the process of systematically varying impedance presented to a device under test, is a robust approach commonly utilised at the beginning of the design phase of an RFPA to characterise the device's performance at fundamental and harmonic frequencies, typically in terms of output power, efficiency, gain and linearity. The work presented by Alsahali et al. is the first work that combines a fast, modulated load-pull system (known as the Mesuro RAPID system) and digital pre-distortion, producing a powerful ET characterisation bench. Achieving this ET characterisation bench wasn't easy, the team had to combine and synchronise a number of different systems. In particular, the efficiency measurement and the time alignment between the RF signal and the tracking signal were especially challenging to characterise. Despite the challenges, the team's Letter shows that they ultimately succeeded in pulling together myriad subsystems to produce a highly useful package. The authors hope that their proposed system will be useful for the future design of ET-PAs by providing a powerful emulation tool. This first example only allows for fundamental load pull, in order to fully investigate the potential of the new measurement architecture, the ability to incorporate harmonics, specifically the 2nd and 3rd harmonics (significant in realising highly efficient PA modes of operation), will be highly beneficial. In addition, it was found that the modulation bandwidth of the Mesuro RAPID load-pull system is limited to 40 MHz. A 5G system will require at least 100 MHz to fully characterise the nonlinearity of RFPAs, which requires at least five times the modulation bandwidth. The group has already begun working on these bandwidth enhancements - watch this (ever expanding) space.