Wrestling the Nonlinear Bear (review of, "X-Parameters") [Book\/Software Reviews]

This book contains many real-world examples and measurements to keep the math approachable and provide evidence for the usefulness of X-parameters. Many of these examples are very impressive regarding just how well the approximations in X-parameters work. For the reader wanting to understand the mathematics behind X-parameters, there are appendices on Volterra series and Hammerstein symmetry as well as many references. The reader is advised to sort out the series notation used by the authors and the implications of all the subscripts from the beginning. Most of the equations you will see will be a variation on this theme. Measuring X-parameters is one thing, but understanding the limitations of the measurements is another. The three longest chapters in this book cover the linearization approximation used for X-parameters, the handling of incommensurate large multitone signals, and extending X-parameters to dynamic and wideband X-parameters to handle memory effects. X-Parameters contains exercises and good examples (Doherty amplifiers, GSM amplifiers, commercial GaN HEMTs, and commercial amplifiers showing memory effects), which could form the meat of an advanced course in measurements. One of the problems with measurements and small-signal assumptions is that instrumentation is really poor at raising its hand and saying the signals are no longer small. Simulators are as bad as instrumentation in telling us we have violated the small-signal assumption. Engineers have to be vigilant with measurements and models to make sure they are close enough to the truth to be useful.