AWR eBooks

RF Electronics Chapter 10: Operational Amplifiers Page 358 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. linear oscillator analysis to determine the oscillating frequency, output spectrum, phase noise, and waveforms, as described in chapter 6. Figure 10.21. Circuits for 10 MHz oscillator, with ideal OpAmps. The changes in performance can be investigated by changing the circuit gain (Rg), the resonator impedance (Zr), the voltage divider ratio Kh = Lr1/(Lr1 + Lr2) and the current limiting resistor (Rs). In some cases, the best performance is obtained with values that are limited by other constraints. For example, reducing the resonator impedance Zr, reduces the phase noise. Reducing Kh also reduces the phase noise. However, it is difficult to make inductors reliably when Lr1 16 nH. Increasing the gain by reducing Rg, increases the output voltage and generally decreases the phase noise. In a practical OpAmp, that also decreases the bandwidth and may prevent oscillation. Optimising Zr, Kh, Rg and Rs to minimise the phase noise, while keeping Lr1 16 nH, results in Zr = 2.5, Kh = 0.4, Rg = 15 Ω, Rs = 680 Ω. The same values, Zr = 2.5, Kc = Cr2/(Cr1 + Cr2) = 0.4, Rg = 15 Ω and Rs = 680 Ω are also used for the Colpitts oscillator. The circuit values shown in figure 10.21 correspond to these values. Figure 10.22 shows the linear Hartley oscillator OSCTEST loop gain versus frequency. The rate of change of the imaginary part of the gain is much higher than that shown in figure 6.9. This should result in a lower phase noise for the OpAmp based oscillators, but they are limited to lower frequencies. Figure 10.22. Linear 10 MHz OpAmp oscillator analysis. The phase noise produced by the OpAmp oscillators depends on the diode models used. Using the MMSD301T1 RF Schottky diodes, results in a low phase noise and ensures that the diodes do not limit the phase noise. Using 3 diodes in series results in a similar RF Electronics: Design and Simulation 358 www.cadence.com/go/awr

• Cover