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RF Electronics Chapter 6: Oscillators Page 175 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. oscillating amplitude is determined. This amplitude is then used in a non-linear Harmonic Balance simulation of the oscillator circuit. This allows the output waveforms, harmonics and the phase noise of the oscillator to be determined. The accompanying NI AWR DE project files for the oscillator designs presented in this chapter are very useful to investigate changes in phase noise performance, output power and harmonic levels that can be obtained by varying different parameters in the oscillators. It is important that the secondary parameter F0 be set to the expected oscillating frequency and that VPMax be set to be slightly larger than the saturated voltage at that node in the circuit. Frequency Selective Networks For typical RF and Microwave oscillators, the following oscillator types are commonly used: Wien Bridge Oscillator; This uses an RC network with a Q of 1/3. This is a low Q resulting in a high phase noise for the oscillator. The oscillator is however easy to construct and tune and as a result this oscillator is often used for audio frequencies, where the inductors required by other configurations would be too large to be used in oscillators. Colpitts Oscillator; This uses a tapped capacitor LC network, and the similarly related. Hartley Oscillator; This also uses a tapped Inductor LC network. Both the Colpitts and Hartley oscillators are similar, in their frequency range of operation, output voltage and phase noise. The LC resonators used in both the Colpitts and Hartley oscillators will have a typical maximum Q of 250. In practice, unloaded Q values of 100 are more common. Voltage Controlled Oscillator; One of the capacitors in the LC network for a Colpitts of Hartley oscillator is replaced with a varactor diode, allowing the capacitance of the LC network to be varied by applying a DC bias. Voltage controlled oscillators are used in many applications. Crystal Oscillator; A quartz crystal is used as part of the frequency selective network, resulting in a very high Q resonator. Fundamental mode crystal oscillators are available for frequencies between 32 kHz and 100MHz. Crystal oscillators have very low phase noise. Quartz Crystal resonators will be discussed in more details later. Stripline or Microstrip Transmission-Line Resonator; At VHF and microwave frequencies, these resonators are a reasonable size and have typical unloaded Q values of 200. Coaxial Cavity Oscillator; The resonators can be made from coaxial resonators with a centre length of one-quarter wavelength. These resonators can cave unloaded Q values of several thousand. The resonators are large, restricting the applications. Dielectric Resonator Oscillator; By filling the coaxial resonator with a material with a high dielectric constant, the linear dimension is reduced by the square root of the dielectric constant. Having a dielectric constant of 25 will thus reduce the volume of the resonator to 1/125 of the air filled resonator volume. Dielectric resonators are used frequently at microwave frequencies. The resonators can be based on the coaxial resonator and have a centre conductive resonator and conductive walls, or they can simply be a high dielectric material, relying on the dielectric-air interface to keep the electric fields inside the resonator. A plot of the Q values of different resonators and their corresponding volume is shown in figure 6.4. The phase noise of an oscillator is primarily determined by the Q of the RF Electronics: Design and Simulation 175 www.cadence.com/go/awr

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