This book focuses on linear, passive structures described by Maxwell's equations or circuit theory. If we combine these elements with active, non-linear components (diodes, transistors) we can design active circuits like oscillators, amplifiers and mixers.
As intensively discussed in the previous sections linear, passive circuits are understood on the basis of electromagnetic fields, propagating waves and linear circuit theory. The design is based on EM simulation or (linear) circuit simulation.
Active and non-linear RF circuits on the other hand are based on general (non-linear) circuit theory. RF electronics require knowledge of basic design concepts for amplifiers, transistor parameters, stability considerations, non-linear behaviour (1 dB compression point, 3rd-order intercept point), noise figures, and so on. RF circuit simulators can usually handle these aspects. However, it is beyond the scope of this book to go into the details of RF electronics. The following list provides sources for further reading: [1, 17–21].
Nevertheless, in the following sections we will provide a short overview of important, active key elements in RF transmission technology to give the reader a first orientation. Figure 6.45 shows a simplified elementary transmitter/receiver unit (transceiver) for wireless communications.
In radio systems the original (low-frequency) signal (e.g. speech, ...