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Frequently Asked Question

Is there any difference between the nature of an oscillator's phase noise and the phase noise from a clock chip?

Yes. There are different factors contributing to the phase noise in an oscillator as contrasted to an amplifier or buffer. An oscillator consists of a resonator with gain and feedback. There has been much research into the complex nature and causes of micro instabilities resulting in oscillator phase noise. Some of these causal factors are random (thermal) noise and active device noise, but there are many other factors, including multiple resonator modes, acoustic effects, flicker noise, etc. The result of these instabilities and noises is to perturb the phase of the signal produced by the oscillator. These phase perturbations have the effect of spreading the clock signal out in the frequency domain with a distinctive spectral density profile. This spectral density profile is interpreted as the phase noise of the oscillator.

On the other hand, the causes of noise in an amplifier or buffer or other non-resonant stage are somewhat different. As a signal passes through these non oscillating stages, random (thermal) noise and active device 1/f noise adds to and subtracts from the amplitude of the signal at each instant. The result is also to spread out the signal in the frequency domain, but with a somewhat different spectral density profile. The result is an additional background of noise in a continuous spectrum which overlays the original clock signal. The resulting spectral density profile can also be interpreted in terms of phase noise.

In both cases, oscillator and clock system, the observed smearing out of the signal in the frequency domain can be treated as phase noise. In the time domain, the power of the phase noise (integrated over an appropriate bandwidth) results in variations in the time domain. This effect is called time jitter.