Correcting memory effects
As well as nonlinear effects, memory effects in the amplifier produce a frequency response: essentially a distortion of the amplitude or phase versus frequency. Until recently, correction of these memory effects has been very complex, with elaborate mathematics such as Volterra series required to describe this response. Today, options like the FSW-K18D DPD measurements extension greatly simplify the process. For every arbitrary signal, the software calculates the frequency response along with the EVM value that describes the signal quality. Instead of approximating with polynomials, it applies iterative approximation via the individual samples. The signal played back by the generator and distorted by the device under test is measured and the pre-distortion is adjusted. After multiple iterations, the signal generator outputs a signal with optimal pre-distortion. In this way, the option compensates both nonlinear distortion and frequency response for a predefined signal sequence. The result delivers the best possible reference for pre-distortion algorithms employed by the user.
The 2 GHz analysis bandwidth now available in the FSW43 and FSW50 makes it possible to characterize wideband amplifiers.
The author, Martin Schmähling is the Spectrum Analysis Product Manager at Rohde & Schwarz, Munich