RF Electronics Chapter 10: Operational Amplifiers Page 361
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 10.27. Phase noise for a 10 MHz oscillator using different OpAmps.
Comparing figures 10.23 and 10.27, show that for Q = 200, the ideal OpAmp Hartley
oscillator at 1 kHz away from the carrier, produces a phase noise of -136.5 dB, compared
with the best phase noise of -132 dB for the Spice models of the OpAmps. Figure 10.29
shows that the AD829 OpAmp slew rate limits and that results in a poor phase noise.
Figure 10.28. Output spectra for the oscillators of figure 10.23.
Figure 10.28 shows the output spectra of the different OpAmp oscillators operating at 10
MHz. Even though all the resonator components are identical, the oscillating frequency
is slightly different due to differences in the input impedance and phase shifts of the active
devices.
Figure 10.29 shows the output waveforms of the oscillators. The AD829, AD8009,
AD8057 and the TSH4022 all have 5 V power supplies and the LMH6629 has 2.5 V
power supplies. The AD829 circuit exhibits slew rate limiting. That is an explanation for
the poor phase noise performance of the AD829 in figure 10.24. Figure 10.29 shows there
is an error in the Spice model for the AD8057, the peak voltage in figure 10.29 is 5.62 V.
Since the circuit runs from a 5 V power supply, the output cannot exceed 5 V.
RF Electronics: Design and Simulation
361 www.cadence.com/go/awr
