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RF Electronics Chapter 6: Oscillators Page 179 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. of the resonator. Figure 6.7 shows the amplifier real output impedance is 19 Ω. That is the impedance used for Port 1 of the resonator. The reactive impedances are chosen to make Lc = 50 nH at 100 MHz, resulting in Cr1 = Cr2 = 101.3 pF as shown in figure 6.8. Those are practical values. The components can be tuned to ensure a 0 (or 180 if needed) phase shift at the required operating frequency. In figure 6.8, the imaginary part of the transfer function is not exactly zero, since the network shown corresponds to the one actually used in the oscillator and allows for phase changes produced by the amplifier. Step 4: Linear Oscillator Analysis The resonator and amplifier are now combined into the one circuit. The element OSCTEST is used to determine the loop gain and phase shift. Set the secondary parameter FC to 160 MHz, which is just above the largest frequency of figure 6.9 and is below the expected 200 MHz second harmonic frequency. The resonator network is now tuned to give a phase shift of zero degrees between port 1 and 2, at the required operating frequency. If needed, the loop gain is changed to ensure that it is just slightly above one at the operating frequency. That then corresponds to the correct conditions for oscillations. To minimise the loading on the amplifier, and that load affecting the Q of the resonator circuit, port 3 is terminated in a 1 k load. If needed, the resonator network centre frequency can be tuned to make the imaginary part of VTG(2,1) = 0. Figure 6.9. Linear analysis using OCSTEST Step 5: Nonlinear Oscillator Analysis Figure 6.10. Circuit for non-linear analysis and corresponding circuit elements. Replace OSCTEST with OSCPROBE as shown in figure 6.10, to do a harmonic balance analysis. Set the secondary parameter F0, to 100 MHz, to make that the initial guess for RF Electronics: Design and Simulation 179 www.cadence.com/go/awr

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