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RF Electronics Chapter 10: Operational Amplifiers Page 362 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 10.29. Waveforms of the oscillators of figure 10.23. Limitation of Simulations Simulation can give a good indication of the expected performance of a circuit. However as indicated in figure 10.9, series inductance and parallel capacitance associated with many resistors can result in a measured hardware performance that is different from the simulation results. It is also important to critically evaluate the simulated results. For example, figure 10.29 shows a peak output voltage of 5.62 V for the AD8057 OpAmp oscillator. That voltage is larger than the 5 V power supplies, and is thus not possible. Some Spice diode models provided by manufacturers, do not include the diode capacitance when they are imported into AWR DE and used in linear simulations. That capacitance varies with the reverse voltage and when these diodes are used for input protection of RF circuits, the effect of that capacitance is critical. The MMSD301T1, RF Schottky barrier diodes, have a capacitance of 1.2 pF for a 5V reverse bias. Figure 10.13 shows that causes a significant gain reduction at high frequencies. Sometimes the simulations raise questions that can only be answered by constructing the hardware and measuring the performance. For instance, in figure 10.20, the AD8057 has a lower bandwidth and poorer noise figure than the AD829. In figure 10.27 it is the other way around. In the light of the AD8057 simulation giving unrealistic output voltages, which one is right? The simulated phase noise for the TSH4022 OpAmp with component values corresponding to figure 10.26, with an ideal resonator is worse than that with a Q of 200. That is not what one would expect. The phase noise of the TSH4022 oscillator in figure 10.27 has a different slope compared to the other oscillators. Are these correct results? The nonlinear simulation using the AD797 OpAmp for figure 10.27, would not converge, even though the linear simulation shows a suitable gain and phase shift for oscillation and figure 10.20 shows nonlinear simulation using the AD797 OpAmp a that its bandwidth is larger than the AD8057. Will this AD797 circuit oscillate or is the expected 10 MHz output frequency beyond its capabilities? RF Electronics: Design and Simulation 362 www.cadence.com/go/awr