Test envelope tracking amps with VSG digital predistortion function

February 14, 2014 // By Graham Prophet
Rohde & Schwarz has integrated realtime digital predistortion into its SMW200A high-end vector signal generator (VSG), an option that the company says shortens test time and simplifies evaluation of envelope tracking amplifiers Predistortion can extend the operating range of amplifiers but determining the right correction factors has always been a time-consuming process.

The SMW-K541 digital predistortion option allows users to import predistortion coefficient tables directly into the SMW200A. The signal generator uses these delta values to adjust the baseband signal in realtime. The option greatly reduces test times since predistorted waveforms no longer have to be tediously recalculated and imported into the generator.

Amplifiers are most efficient when they operate near their maximum output power, which occurs in the nonlinear range. Although more power efficient, this results in distortions at the amplifier output, which considerably diminishes the amplifier's RF performance. Developers, especially developers of envelope tracking power amplifiers for smartphones, base stations and tactical radios, use digital predistortion to counteract this problem. In envelope tracking, the supply voltage tracks the envelope of the RF signal. Envelope tracking can optimise efficiency, but is a valid design approach only when predistortion is used to compensate for nonlinear effects.

Users import an AM/AM or AM/PM table with correction factors directly into the VSG. The SMW200A uses the table values to automatically calculate in realtime an amplitude and phase offset for each I/Q sample. The original waveform file remains unchanged. Conventional solutions require users to import a new, predistorted waveform file into the signal generator each time the level changes.

Developers can use the digital predistortion option on all communications standards supported by the R&S SMW200A as well as on their own custom waveforms. For envelope tracking tests with predistortion, the digital predistortion option can be configured and seamlessly combined with the R&S SMW-K540 envelope tracking option so that the envelope signal can be generated either from an undistorted or predistorted signal.

Rohde & Schwarz; www.rohde-schwarz.com