Envelope tracking in amplifier testing simplified
Amplifiers are most effective if they are operated near maximum output power which means they are operated in the non-linear region of their characteristic. This however generates significant distortion at the output which in turn has a negative impact on the RF performance. To counteract, developers utilise digital predistortion. This technique is particularly popular among designers of power amplifiers with envelope tracking for smartphones, cellphone base stations, and tactical radio stations. Envelope tracking means that the supply voltage follows the envelope curve of the RF signal. This optimises the energy efficiency of the amplifier; however, it makes it necessary to compensate for non-linear effects by means of a defined predistortion.
The R&S SWM-K541 Digital Predistortion option enables users to load the correction values by means of an AM/AM and AM/PM table directly into the R&S SMW200A Vector Signal Generator. Using the values in this table, the generator then automatically computes the amplitude and phase offset to each I/Q sample. The initial waveform file remains unchanged. With conventional methods users need to calculate a new, undistorted waveform file after each change of the level. This file than must be uploaded to the signal generator.
Developers can apply the Digital Predistortion option to all communications standards available on the SMW200A as well as to their own customer-specific waveforms. For envelope tracking tests in conjunction with predistortion, the option can be connected seamlessly with the Envelope Tracking option and configured flexibly. Thus they have the choice of generating the envelope signal either from the undistorted or the distorted signal.
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