RF Electronics Chapter 6: Oscillators Page 190
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 6.28 Output waveforms of Colpitts and Hartley low noise oscillators.
Crystal Oscillators
Crystal have very high Q values, so that crystal oscillators have much lower phase noise
than oscillators with LC resonators. For details of Quartz Crystals see "Quartz Crystal
Resonators and Oscillators for Frequency Control and Timing Applications - A Tutorial"
[6] by the 2009 IEEE president John Vig. He has graciously given permission for his
slides to be used as a resource for this book.
Example 6.2: Crystal Oscillator Design
The design of a crystal oscillator is similar to that of any other oscillator. For most
fundamental mode crystal oscillators, the crystal is inductive at the operating frequency
and the inductor in any LC oscillator is replaced with the crystal. In figure 6.29, the
inductance of a phase shift oscillator is replaced with a quartz crystal. The inductance is
calculated from the crystal's frequency Fs and capacitance C1. A minor frequency change
for Fs is made for the resonance shown in figure 6.34 to be exactly at 25 MHz.
Figure 6.29. Phase shift network using a quartz crystal.
The amplifier shown in figure 6.30 has a gain of 25 dB from the input port 1 to the output
port 3. The gain from the input to port 2 is enough to cause oscillations. The gain of the
amplifier can be controlled by changing R4, the resistor connected to the emitter, without
affecting many other parameters. The resonant network and the amplifier are connected
together to form a Crystal oscillator as shown in figure 6.31. Using OSCTEST, the gain
around the amplifier and resonator loop is now determined using a linear oscillator
analysis as shown in figures 6.31 and 6.33.
RF Electronics: Design and Simulation
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