Which type of VCO is generally used in PLL design?

[Analog_IC]

VCO architecture

Which type of VCO (LC or Ring) is generally used in PLL design. Does it depend upon the application of IC.

 

[Teddy]

Re: VCO architecture

My approach would be ring - lower freq and higher jitter.
LC - higher F (GHz) and subpico jitter.

With ring I am able to get bellow cca 5ps jitter but for anything less than 2ps I woul ddefinetely go for LC tank.
ALso ring will not be as fast.

So yes it does depend on application or better spec of IC

 

[Analog_IC]

Re: VCO architecture

Thanks,
How do we measure jitter of PLL(lock condition){I mean before fabrication }. I could measure jitter of a free runing VCO using cadence-spectre tool.

 

[Teddy]

Re: VCO architecture

Honestly I don't do it. I do not have good enough (noise) models for my devices to be able to compare it to real thing.
But you can for sure do eye diagram. I guess you would obtain at least reference to which you can compare in case you do some changes.
I was never able to simulate jitter good enough to rely on it. But that's me.

 

[khouly]

Re: VCO architecture

it depends on the application , and system architecture of it

in the systems that require low phase noise it is prefered to use LC VCO
if the phase noise limitation not so strict , u can use ring specially it takes low area
on sillicon , and it is very simple to make it differential and get I , Q
in LC if u need I,Q u will need at least 4 spirals , which mean more area

thanks
khouly

 

[yan2004]

Re: VCO architecture

1. Ring oscillators are normally used for low frequencies oscillation.
2. Phase noise in closed loop conditions can not be easily calculated. However, it can be approximated with some assumptions. One of a typical calculation for phase noise can be found in the paper “Prospects of CMOS Technology for High-Speed Optical Communication Circuits” by Razavi

 

[cjg40]

Re: VCO architecture

i think ring vco has higher Q than lc vco,so ring vco has higher stabilitier than lc vco,but it has lowerfrequency than lc vco.

 

[Han]

Re: VCO architecture

Ring oscillators CAN work even at 5GHz in 0.13u process.
The problem is the generated noise of the VCO, so if one can afford BW large enough and not concerned about jitter transfer, I would definitely choose the ring oscillator.

 

[Teddy]

Re: VCO architecture

Regarding the speed of VCO - I did ring osc running 3.5GHz on 0.18umCMOS which was < 5ps jitter.
LC you can do way faster and way lower jitter (< 1ps)

Ring - smaller and easier to make if one can live with the performance.

 

[dumbfrog]

Re: VCO architecture

analog => ring type
rf => lc type

 

[paladinzlp]

VCO architecture

10M~800MHz => ring type
>1G => LC type