VCO and frequency divider

[calculus_cuthbert]

differential + divider

Hi,

I am designing a differential LC VCO and want to divide the frequency down. I had a couple of questions regarding the VCO design and the design of a frequency divider.

Questions:

1. What is the advantage of having a differential VCO? Does it improve the phase noise?

2. How do I connect the differential VCO to a frequency divider because the ferquency divider has a single ended input? (I need to convert the differential output to a single ended output?)I was thinking (please correct me if I am wrong): If I have enough gain so that the output of the VCO goes from rail to rail then I can connect the differential output to a difference amp with minimum gain, and then connect the output of the difference amplifier to an inverter. The output of the inverter goes to a frequency divider.

3. If it is not important to use the differential output of the VCO then we can just connect one of the outputs of the VCO to a frequency divider.

4. Is it necessary to design an analog frequency divider or it is fine to design a digital frequency divider?

I would really appreciate it if someone could help me out.

Thank you.

 

[saro_k_82]

differential vco

1. A differential VCO doubles the signal swing thereby increasing the carrier power. So Phase noise improves. Moreover the substrate noise gets canceled within the VCO., does not introduce current spikes on the supply line, low harmonics and lower supply pushing.
2. The load that the VCO sees must be symmetric as well., or else the imbalance will show up as phase noise (and in other parameters as well). Your idea works., but such a stage will have different input impedances for VCO frequency at the two inputs.
3. You lose on phase noise. It is better to use atleast one or two CML dividers, so that you don't introduce high frequency substrate noise. In most cases this solution is justified by power consumption. It also becomes easier to convert to CMOS after division.
4. Depends on your frequency output.

 

[calculus_cuthbert]

frequency divider site:edaboard.com

Thank you for your reply. It was very helpful.

Could you please suggest another way to connect the frequency divider to the differential VCO?

What would be the simplest way to do this?

 

[saro_k_82]

vco frequency devider

www.truecircuits.com/images/pdfs/maneatis96b.pdf
This has one nice idea in the end., but the two differential stages can be one single differential gm/gm stage if the signal swing is large enough.