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RF Electronics Chapter 6: Oscillators Page 187 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. the Impedance for the three different coil configurations. The Q of the Coil element is ��� at ��� ���. � near �alue Q � ��� is used for con�enience in the follo�ing designs. Any vibration will slightly change the inductances, that will frequency modulate the oscillating frequency with the vibration. That is called microphony. Putting the coils on a solid former reduces this microphony. The element MCINDN is uses a spiral Microstrip inductor without a bridge for the connector at the centre of the spiral. Using MCINDN on an RO4003 substrate, with 1.5 turns, a track width of 1.5 mm, a spacing of 0.2 mm and centre wire radius of 3.016 mm results in the same 16 nH inductance as the coil above. However, figure 6.23 shows that a lower Q of 93.8 is obtained for the MCINDN coil. One can also use a length of Microstrip transmission line as an inductor, that allows smaller "inductors" to be made, but they need to be simulated as transmission lines to obtain the correct circuit performance. Figure 6.23. Element COIL, INDQ and MCINDN impedances. Example 6.1: Low Phase Noise Oscillator Design Low Phase Noise Colpitts Oscillator The Q of the resonator is a prime factor in the phase noise of the oscillator. With practical limits of Zr = 10 ٠and Q =200, the effective Q of the resonator can be maximised by minimising the loading on the resonant network. The output impedance of the emitter in figures 6.10 and 6.19 is a low impedance that reduces the effective Q of the resonator. By placing a resistor Rsc in series with the resonator, the effective loading of the resonator can be reduced, resulting in a reduction of the phase noise. In addition, the performance of the active devices will affect the phase noise of the oscillator. Tuning the different components of the circuit allows the phase noise to be minimised. Whether this also results in a minimum phase noise of the hardware, depends on how good the simulation model of the transistor matches the hardware. For the low noise Colpitts Oscillator, the circuit of figure 6.10, is used as a starting point. Lowering Zr to 10 ٠reduces the resonator loading of the base and emitter of the transistor and thus reduces the phase noise. Reducing Zr further, will reduce the phase noise, but then require inductors that are very difficult to realise. Adding a voltage follower to buffer the output prevents the load connected to the oscillator from changing (pulling) the oscillation frequency. Tuning Kc, the ratio of Cr1 and Cr2, so that the total capacitance RF Electronics: Design and Simulation 187 www.cadence.com/go/awr

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