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RF Electronics Chapter 6: Oscillators Page 182 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. By including a SWPVAR simulation control (Element Simulation Control SWPVAR) in the schematic of figure 6.10, as shown in figure 6.14, the effect of variations of inductor values on the oscillating frequency can be investigated. For this to work, the value Lc must be included as a fixed variable, as shown in figure 6.14 rather than Lc being calculated as shown in figure 6.10. The resulting plot is shown in figure 6.15. Figure 6.15. Change in oscillating frequency as the resonator inductor varies. The oscillating frequency can be set by winding the inductor L1 on a coil former and using a ferrite tuning-slug to change the inductance of the coil. A varactor diode allows electronic control of the frequency or frequency modulation of the oscillator. By placing a varactor diode either in parallel with Cr2 or in parallel with inductor L1, in figure 6.8, together with suitable biasing allows the frequency of the oscillator of figure 6.10 to be controlled by varying the reverse bias voltage across the varactor diode. The above design procedure allows the performance of an oscillator to be accurately determined prior to the hardware being produced. This greatly reduced the cost of designing oscillators. The non-linear analysis can only be produced when a full Spice model of the active device is available. Many transistor manufacturers provide the required models for their transistors. Unfortunately, the SPICE models are not available for many commercially available Microwave Monolithic Integrated Circuits (MMIC). The same design process can be used for oscillators using the Operational Amplifiers described in chapter 10. Hartley Oscillator A Hartley oscillator uses a tapped inductor, while the Colpitts oscillator uses a tapped capacitor. There are different ways that the resonator can be connected to the transistor. The transistor can be arranged as a common emitter, a common collector or a common base amplifier. A Hartley oscillator is typically ��� made using a common emitter transistor as shown in figures 6.17 and 6.18. The resonators are arranged to minimise their loading. The input is connected to the collector, which is close to a constant current source, and the output connected to the base of the transistor, which presents a high impedance. In addition, the biasing of the transistor in figures 6.17 and 6.18 are arranged so that it does not load the resonator. To be able to optimise the designs and keep the same circuit values for both the linear and nonlinear circuit simulations, the tuneable parameters and equations shown in Figure 6.16 are used. RF Electronics: Design and Simulation 182 www.cadence.com/go/awr

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