Phase Noise of a circuit

[kenambo]

Hi

Do you think phase noise occurs only in PLL /VCO ? If so what causes this phase noise for this circuit and why we measure Jitter instead of Phase Noise in other circuits?

I have read many papers regarding these noises like thermal noise, shot noise flicker noise.. and how these influences phase Noise in a Oscillator?

The digital switching also causes noise. But how it causes noise? and why we call it as switching noise?

Thanks.

 

[BigBoss]

You can imagine a water sea that has a ideal sea level and this is constant (no perturbation) .When the rain starts,this level will not be constant any more and its sea level will show some small differences compare to before.
Phase Noise looks like that.Noise vector is added to the main vector ( carrier ) and it creates phase fluctuations and amplitude fluctuations ( much smaller compare to phase ).You can imagine also that there are noise sources/generators in a circuit and these generators are somehow connected to the main generator.The main generators waveform will absolutely more or less impacted by these noise sources.

 

[kenambo]

hi..

So In PLL if phase noise occurs and the phase varies then it will be corrected by the PFD is that right?

or the phase doesn't vary according to phase Noise..

Please clarify this..

Thanks

 

[Taher_Selim]

Hi
I wanted to add that in general phase distortion occurs due to non-linearity in phase response of the circuit. i.e if the phase response of the circuit is linear, it means that d(PHI)/d(OMEGA) is constant, so any extra frequency superimposed on the original signal will have same phase change, so the overall phase is constant before and after the circuit.
If the i.e if the phase response of the circuit is non-linear, it means that d(PHI)/d(OMEGA) is changing according to the superimposed frequency.

That means that we have phase distortion in many systems, VCO, PLL, Filters,...

Regards

 

[kenambo]

So what i am thinking is if we take oscillator as a single circuit the phase noise can cause some errors But as a whole PLL It doesn't increase because we have pfd in the PLL loop.

Am i right?

 

[BigBoss]

hi..

So In PLL if phase noise occurs and the phase varies then it will be corrected by the PFD is that right?

or the phase doesn't vary according to phase Noise..

Please clarify this..

Thanks

No, Phase Noise is a fluctuation on the phase of the signal.PFD in a PLL supplies only correction currents ( I presume it's equipped with charge pump) regarding to phase difference between ref. clock and main clock.You confuse Phase Noise and Phase Differences.They are not same things.Phase Noise can not be controlled and it's absolutely random small fluctuations on the phase of the carrier.PFD cannot correct these small fluctuations..