RF Electronics Chapter 10: Operational Amplifiers Page 360
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 10.24 shows the spectra for the Hartley and Colpitts oscillators with ideal OpAmps
and LMH6629 OpAmps and using the same component values as shown in figure 10.19.
For the ideal OpAmp and the LMH6629 OpAmp circuits, the fundamental output is
nearly the same for the respective Colpitts and Hartley oscillators. The Hartley oscillators
have a low even harmonic distortion. Figure 10.25 shows the corresponding waveforms.
Using 3 diodes in series, results in a similar output from the ideal OpAmps compared
with the LMH6629.
Figure 10.25. Waveforms of Hartley and Colpitts oscillators.
Figure 10.26. Circuit for 10 MHz oscillators, using LMH6629 OpAmp.
To compare the phase noise from oscillators using different OpAmps, the circuits of
figure 10.26 are used, with the same component values but different OpAmps and supply
voltages to match those required by the OpAmp. The circuits are the similar to figure
10.21. The resistor Rs prevents the low output impedance of the OpAmp from loading
the resonator. For comparison, the values of Zr , Kh , Rg and Rs, are kept the same as for
figures 10.21 to 10.26. Figure 10.24 shows the phase noise of these oscillators and as a
reference the LMH6629 Hartley oscillator shown in figure 10.23 is again included. The
AD829 and TSH4022 OpAmps are capable of operating at 15 V. That then requires the
secondary parameter VpMax of OSCAPROBE to be set to 15 V for the simulation to
converge. The phase noise of the AD829 does not change much as the supply voltage
changes. The TSH4022 gives a much lower phase noise at a 5 V supply. As a result in
figure 10.27, a 5 V supply has been used for both the TSH4022 and AD829.
RF Electronics: Design and Simulation
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