Which will have less phase noise, PLL or VCO?

[rajanarender_suram]

PLL Vs VCO

In a locked condition of PLL and a stand alone VCO (say LC VCO) give same output frequency than which will have less Phase noise and why?

 

[ieropsaltic]

Re: PLL Vs VCO

First, at locking condition at PLL, the output frequency is compared to a very stable reference (crystal oscillator) using feedback so the PLL's output frequency is more stable than stand alone VCO.
Concerning phase noise, the output of PLL will have less phase noise due to VCO than stand alone VCO due to the high pass filtering transfer function seen by VCO phase noise at PLL. It should be noted that PLL have other noise sources besides VCO but most of them see low pass filtering transfer function.

 

[rajanarender_suram]

PLL Vs VCO

I think VCO output when gone through the loop i.e when returned to the same point( output gone through freq divider, PFD, Charge-pump and returned at the input of VCO ) it looks as HPF, but we are taking the output of VCO directly from the PLL.

 

[ieropsaltic]

Re: PLL Vs VCO

When modeling the VCO at PLL, it's modeled as Kv/s where s is la place's operator and phase noise of VCO is modeled as noise source ADDED to the output of the VCO. This leads to HPF transfer characteristics at the output when the loop is closed.
You can refer to "RF microelectronics "BY Razavi chapter 8 pages 266-268 for more details.