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RF Electronics Chapter 6: Oscillators Page 205 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Simulation" in chapter 7 of this book and will not be dealt with here. The design techniques presented in this chapter will allow low noise RF and microwave oscillators to be designed. For a low phase noise, it is important to minimise the external loading on the resonator. Measuring the group delay as part of the OSCTEST gain and phase adjustment gives a good indication of the phase noise performance. Phase Locked Loops In practice RF oscillators are used in transmitters and receivers. As a result, the oscillation frequency must be precisely controlled to satisfy the relevant communication system regulations. To enable the frequency of an oscillator like the one in figure 6.9 or 6.24 to be electronically controlled, a varactor diode is used as an electronically variable capacitor and placed in parallel with either Cr2 or in parallel with Lc. Then changing the reverse voltage across the varactor changes the frequency of the oscillator, so that the oscillator is a voltage-controlled oscillator (VCO). The block diagram of a phase-locked loop is shown in Figure 6.55. It consists of VCO, and a loop filter which primarily determines the transient behaviour of the phase locked loop, but which must also attenuate any high frequency components produced by the phase detector. The reference input can be the output from a Crystal oscillator, like the one of figure 6.31. When the PLL is locked, the output frequency is N times the input frequency. The type of Phase Detector used, as well as the loop filter and VCO determines the behaviour of the phase locked loop. ���� ��� � � � ���� ���� ���� Eqn. 6.3 N is the divider ratio, K is a gain = KpKv, where Kp is the phase detector transfer function and Kv is the VCO Hz/V constant. H(s) is the loop filter transfer function and s is the complex frequency variable. The PLL transfer function is a low pass function, so that at low frequency, the phase noise is dominated by the crystal oscillator noise, multiplied to the VCO frequency [9, 10]. At frequencies above the natural frequency (loop filter band-with) of transfer function of equation 6.3, the VCO noise dominates. Typical phase detectors are multipliers or mixers described in chapter 5, exclusive OR gates or phase-frequency detectors [11]. Phase frequency detectors provide a feedback signal when the VCO is not locked, and will thus provide more reliable phase locking of the VCO. All these phase detectors produce unwanted signals at harmonics of the Reference Input frequency. Those must be filtered out to produce a clean output spectrum. The natural frequency must thus be much lower than the reference frequency input. Figure 6.55. PLL Block Diagram. RF Electronics: Design and Simulation 205 www.cadence.com/go/awr

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