Digital pre-distortion compensates nonlinear effects in amplifiers
Component testing requires instruments with the appropriate measurement applications, such as the FSW-K18 firmware option to comprehensively characterize amplifiers, and the new FSW-K18D direct DPD measurements extension that makes it easier to compensate for memory effects.
Power amplifiers in base stations or smartphones must demonstrate high linearity over a wide frequency range to deliver the exacting transmit and receive characteristics. Unwanted nonlinear effects, however, generally occur in the upper power range and diminish signal quality. They manifest themselves as higher EVM values and increased interference in adjacent channels. Possible consequences are lower orders of modulation and – hence lower data rates – along with signal interference in adjacent channels.
Effective characterization of these effects makes it possible to provide digital compensation. Here, the signal is digitally pre-distorted upstream of the amplifier to counteract the distortion of the amplifier. This eliminates nearly all the artefacts caused by the amplifier, so that the developer receives an almost linear signal at the output of the amplifier or mixer.
Amplifier measurement applications that integrate functions for digital pre-distortion can be used to comprehensively characterize distortions caused by nonlinear output amplitudes or phase changes relative to the input signal (AM/AM and AM/PM). It is also possible to check the effects of pre-distortion. For example, the FSW-K18 initially compares an ideal reference signal from the signal generator against the acquired and distorted readings. Using this data, it generates a polynomial function that describes the pre-distortions by way of approximation then transmits corresponding correction data to the signal generator, which in turn generates the pre-distorted signal. Analysis bandwidths of triple, quadruple and quintuple the signal bandwidth are typically used in this process.