RF Electronics Chapter 6: Oscillators Page 192
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 6.33. Frequency response for the oscillator using linear oscillator analysis (OSCTEST).
Replacing the OSCTEST element with the OSCAPROBE element as shown in
figure 6.31, allows a harmonic balance analysis to be performed. Figure 6.34 shows the
output spectrum, figure 6.35 shows the corresponding waveforms, and the phase noise is
shown in figure 6.36
Figure 6.34. Output spectrum obtained using harmonic balance analysis. (OSCAPROBE)
The oscillating frequency can be varied by typically tens of parts per million, by placing
a varactor diode either in parallel with C1 or C2 in figure 6.29 and controlled the
frequency by varying the reverse bias voltage across the varactor diode. The oscillator
then is a VCXO (Voltage Controlled Crystal Oscillator). Such control is used to obtain
the precise frequency setting required for radio transmitters.
By comparing figures 6.13, 6.22 and 6.26 with figure 6.36, it can be seen that the crystal
oscillator has a much lower phase noise, due to the Q of the resonator being much higher.
The phase noise at 25 kHz in figure 6.36 is determined by the resistor and transistor noise
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