SUMMARYThis paper presents the design method, properties and driving techniques of a linear 0.5 W 1 GHz LDMOS power amplifier used in a supply modulated envelope tracking transmitter with real-time predistortion. Causes of nonlinearities are identified, and various supply voltage drives are experimented. Measured results show that the power efficiency can be improved while maintaining high linearity.

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Severe shot peening (SSP) was used on additive manufactured 316L by laser powder bed fusion. The effect of the post processing on the surface features of the material was analyzed through residual stress measurements, tensile testing, hardness-depth profiles, and fatigue testing by flexural bending. The results showed that SSP can be utilized to form residual stresses up to −400 MPa 200 μm below the surface. At the same time, a clear improvement on the surface hardness was achieved from 275 HV to near 650 HV. These together resulted in a clear improvement on material strength which was recorded at 10% improvement in ultimate tensile strength. Most significantly, the fatigue limit of the material was tripled from 200 MPa to over 600 MPa and the overall fatigue strength raised similarly from a low to high cycle regime.

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Laser weldability of dissimilar parts produced with different techniques from 316L material was studied in this paper. Butt joint welding was performed on pairs of sheet metal and parts produced with additive manufacturing by laser powder bed fusion. The effect of heat treatment was also considered and the stage in which it was applied. The experiments showed very good weldability and the results were verified with microstructural analysis and tensile testing with digital image correlation equipment. Microstructures of the welds revealed morphology consisting of equiaxed and columnar grains in somewhat random orientation. Tensile tests showed that the 75% higher yield strength of the printed material compared to sheet metal leads to uneven elongation of the dissimilar joint and the part breaks from the sheet metal side. Hardness measurements showed higher values for the printed side (225 HV) compared to sheet metal side (170 HV). All the welded specimens broke outside of the welds showing a good weldability independent of the heat treatment stage.

Power amplifier (PA) efficiency is a criticalfactor in any portable device. Two ofthe most popular architectures to improve the efficiency, envelope tracking (ET) and envelope elimination and restoration (EER), are discussed in this paper. A test systemfor characterization ofsuch a system is presented. A Class E based EER system is tested and characterized and software corrections are implemented to linearize the transmitter Test results show an ACPR improvement of 15 dB by only tuning the phase signalk amplitude and adding AM/PM correction. EVM improves from 2.8% to 0. 71% at the same time.

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