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RF Electronics Chapter 2: Computer Simulation Page 27 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. This technique is very useful for obtaining the correct length when the stub has bends in it, as is the case for the design of a Branchline coupler with folded lines to reduce the size, as described in Chapter 4 " Transmission Line Transformers and Hybrids" of this book Nonlinear Time Based Simulation Many electronic circuits are nonlinear. For example, frequency mixers described in chapter 5 use nonlinear properties of diodes to produce the frequency shifting of RF signals used in most radio and TV receivers. The output amplitude of oscillators described in chapter 6, is determined by the saturation of the active devices. Power amplifiers, described in chapter 9, are normally operated at power levels where the active devices are operating close to saturation and are thus operating in nonlinear regions. The ability to analyse the effect of the nonlinear operation of circuits is thus an important part of any design. There are two types of nonlinear time-based simulation. The first type is time domain based transient simulation, like the classical Laplace transform based circuit analysis for linear circuits. The time waveforms produced by computer simulation programs such as Multisim, Microcap and LTSPICE use SPICE time based simulation. AWR DE includes transient time based simulation using APLAC or the optional HSPICE and Spectre software. Transient analysis, begins at time t = 0 and assumes that only DC sources are connected to the circuit to be analysed, before that time. At time t = 0 a stimulus is applied and the resulting current and voltage waveforms are determined at ports or test points. The transient simulators start with the largest possible time increment and reduce that time increment automatically if the results are not accurate enough. If needed the automatic time increment selection can be overridden. See Basic_Transient.emp project in the MWO/AO /Examples [24] for an example included with the AWR DE software. It is desirable to read the help file AWR DE Simulation and Analysis Guide > Transient Analysis > Transient Analysis Basics (Section 7.1) [25], included with AWR DE software for additional information if needed. These help files contain much more detail than what is needed for this book. Transient analysis is useful for determining the transient time and voltage waveforms of a circuit. The second type is Harmonic Balance (HB). This is a frequency based, steady state analysis that uses non-linear differential equations to model the devices and then uses a linear combination of excitation tones to balance currents and voltages to satisfy Kirchhoff's law. The inputs applied to the circuit are one or two tone signals, whose amplitudes or frequency can be swept if needed. The amplitude, phase and frequency at the ports or test points in the circuit, as determined by HB can be used to calculate the corresponding time waveforms. HB has a lower noise floor than transient time simulation and is thus better for determining harmonics. Examples of harmonic balance simulations are shown in Chapter 5 "Mixers" and Chapter 6 "Oscillators" of this book. Harmonic Balance provides a fast simulation of the most common RF circuit analysis; the steady state operation of non- linear circuits, and it is able to easily display the corresponding time waveforms and spectra. Harmonic Balance is thus used more often than transient time domain simulation. AWR DE comes with a built-in HB analyser and more recently with the APLAC TM simulator, which includes both HB and transient analysis simulation. RF Electronics: Design and Simulation 27 www.cadence.com/go/awr

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