What's the main criterion for choosing number of stages in a ring oscillator design?

[chacha]

What is the main criterion to choose number of stages while designing ring oscillator??
Say my VCO operating range is 400M to 800M. How can i choose the no of stages??

Thanks.

 

[khouly]

Re: ring oscillator

u know from the basic equation of the Ring oscillator that the F depends on the N * the delay of the single inveter

where N is the number of stages , so u should check the propagation delay of the inverter and then use it to get the number of stages required to get this frequency

if u want a ring VCO ,, then u should see how will u vary the propagation delay of the inverter

another way to make a broad band VCO is to change both the propagation delay and the stages

like fine and corras tuning

khouly

 

[chacha]

Re: ring oscillator

Thanks Khouly.
Say i am using a 5 stage ring oscillator and i am getting all the frequencies i want.

Can i say anything like 7 stage oscillator is much better??
Is there any concern like 7 stage oscillator is more stable than a 5 stage one??

Thanks...

 

[faizalism]

Re: ring oscillator

Dear Chacha,

Actually it depends on your design parameter, let say u need low power VCO, so u can used least number of cell, but the drawback is the gain. Actually for ring oscillator there is Barkhausen Criterion for get reliable oscillator.

thanks,

 

[khouly]

Re: ring oscillator

i donot think so , it is ok to use 5 stage
ring oscillator require a odd number of stages if ur using single ended inverters

if the 5 stage give u all the range u need then it is ok , u don't need to make the circuit more complex to change the number

khouly

 

[chacha]

Re: ring oscillator

Thanks for your quick responses.
One of my friends told that it is always better to use higher no of stages. But he doen't know the reason.

I too feel that if iam getting gain, frequency range .. i dont have to make it complicated choosing more stages.

Thanks.........

 

[khouly]

Re: ring oscillator

as u increase the stages the gain increase , but the BW is decreased coz the more capacitance associated with the increase number of stages

khouly

 

[ravitest]

Re: ring oscillator

ring oscillator design is given in following book

cmos circuit design, layout and simulation by baker

 

[survivor]

ring oscillator

Hi,

In terms of power and speed, less stage is better.But usually VCO gain of less-satge design is larger compared to more stage design. In terms of VCO phase noise, we don't want VCO gain too large. Another consideration is more-stage VCO is more sensitive to power-supply noise than fewer-stage. So here are some trade-off depends on your PLL architecture and application.


Hope it helps