RF Electronics Chapter 6: Oscillators Page 206
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
When an oscillator is phase locked, the noise below the natural frequency of the transfer
function of equation 6.3 is a little bit more than N times the crystal oscillator noise. The
extra phase noise is due to noise produced by the frequency divider and the phase detector.
It is possible to simulate that [10] but that is outside the scope of this book. Figure 6.56
shows the phase noise of the 100 MHz oscillator of figure 6.24 and the 25 MHz crystal
oscillator of figures 6.29 to 6.36. The green dotted curve is the phase noise of the crystal
oscillator multiplied by 4 to go from 25 MHz to 100 MHz. The red curve is the typical
phase noise of the phase locked 100 MHz oscillator. (Note figure 6.56 is for illustration
only, and is not based on measurements or detailed simulations).
Figure 6.56. Noise from a phase locked oscillator.
Frequency Locked Loop
There are some applications where phase locking is impossible, but where a frequency
detector (or phase frequency detector) is used instead of a phase detector. In that
application, the frequency of the divided VCO signal, is made to lock to the frequency of
the reference input, resulting in a frequency locked loop. Typical examples of this are: 1)
Frequency locking a crystal oscillator to the 1 pulse per second output from a GPS
receiver. 2) Frequency locking an oscillator that is used for FM broadcasting to a
reference crystal. The combination of these will allow an FM transmitter to have a carrier
frequency accuracy better than 1 part per million. Under those conditions, the phase noise
of the locked VCO is the same as the phase noise of the unlocked VCO.
References
1. J P Silver "Oscillator Resonator Design Tutorial", Oscillator Resonators,
https://fdocuments.net/document/oscillator-resonator-design-resonator-design-
tutorial-j-p-silver-varactor-in.html?page=1
RF Electronics: Design and Simulation
206 www.cadence.com/go/awr
