Topologies for 1 GHz VCO design

[khorlipmin]

My work need me to compare different topologies, and implement the best to provide 1GHz VCO with low power and low jitter.

I am looking at 3 topologies:

a) transmission gates + inverter VCO
b) current starved VCO
c) source coupled VCO

so when you said differential pair VCO, do you mean (c) source coupled VCO?
And can anyone with any information on these topologies please give me a shout? Cheers!

 

[STOIKOV]

Re: VCO design

hello, a differential vco can be built with differential pair as shown in the picture

 

[pk3316]

VCO design

differential pair VCO is one type of ring VCO it can be used even stages for ring VCO

 

[montage3000]

VCO design

i think B is ok, becaust its swing is large

 

[khorlipmin]

Re: VCO design

Thanks very much for the diagram. My work only allow me to use CMOS transistor for modelling vco, what is your recomendation then?

 

[Karthi]

Re: VCO design

hi,
The main purpose of using the Differential VCOs is that they are more robust to power supply and substrate noise. But while designing current-starved VCOs special care is to be taken to keep the voltage swing constant. This depends on the control path. It may be a single-ended or a differential path. If the control path is single-ended, u need additional cicuitry ( ex: self-replica bias ) to maintain a constant voltage swing.

 

[khorlipmin]

Re: VCO design

Thanks Kathi,

Why is that single ended Osc uses odd stages while in Diff Osc we can use even stages? Or am I right about this?

 

[jcpu]

Re: VCO design

Hello

Because we can switch +/- in differential type
as in attachment to obtain 180 degree phase shift.
Wheareas in single end type, that can only be obtained
by odd stage of invertors.

Have fun.

 

[khorlipmin]

Re: VCO design

Thanks jcpu for your drawing. So does it mean differential type shouldnt have odd stages? Since with even stages it manages to output 180 phase shift.

 

[Karthi]

Re: VCO design

Hi,
U can use odd number of stages also for differential VCO. See..When u calculate the frequency generated by a ring oscillator, u will see the period as 2 * ( N * td )...This means that the delay of the cells contribute for 180 degree phase shift and the inversion at the final stage contributes 180 degree phase shift. When u use inverters as delay elements, u need odd number of cells because only odd number of cells will give u 180 degree phase shift. In the case of the differential stages (in most of the designs) if u see the successive stages, + -> - and - to + will be connected except for the last stage where + -> + and - to - are connected. Basically, most of the differential delay elements give a phase shift of 180 degree. Basically, a differential inverter..u can say that..so, instead of swapping the connections in the succesive stages, if u swap the connection at the last stage alone, u can use odd number of differential delay elements. I hope the explanation is clear..(correct :?: )..

Regards..
Karthi...

 

[michaelhan]

Re: VCO design

something else what I want to say is that different stages diff Osc have thier own
requirements for phase shift and gain of each stage; usual 3 to 5 stages is enough!

 

[xischaune]

Re: VCO design

i would support a current started VCO
because of its large swing...

 

[jcpu]

Re: VCO design

Be aware of large number of stages,
very likely to give rise to spurious oscillation mode
if layout + postsim not extremely careful.