Help me with design problems in ring VCO design

help me about VCO design

hi,
I have met a troublesome problem when I design a ring VCO that includes 2 parts--control voltage to current and current to frequency. The output frequency changes with the process corner and temperature. I think main reason is the structure of control voltage to current, which is a nmos follower and a resistor. Under different process corner, the resistor changes about 20% in my process model, so the output frequency also changes.
I think the circuit of converting control voltage to current has a few requirements：
1. the linearity--which ensures the linearity between control voltage and output frequency
2. the stability under different temperatures
3. the stability under different process corners
4. the stability under different power supply voltages
If different PVT can affect the current remarkably, then the performence of VCO is not good enough inevitably.
So who can help me to obtain a "perfect"(relative) structure of voltage-->current in VCO? Any useful information or reference are appreciated!
Thanks a lot in advance!
Best regards,
flyinspace

help me about VCO design

what is ring VCO?

Re: help me about VCO design

Three suggestions:
1.In your nmos follower, you must make the nmos
transistor W/L wider and the resisitor bigger,
so Ictrl= Vcrtl/R.
2.A lineraized DAC can helpful,which is proposed
by M. Steyaert,which you can refer his paper
about synthesizer.
3.What you want is the total function of V-I and
f-I is linear,so f-V is linear.Just make the
total function linear, which ease your problem.

Re: help me about VCO design

u can use full differential V-I converter. it's linearity will be better.

Re: help me about VCO design

ericzhang said:

Three suggestions:
1.In your nmos follower, you must make the nmos
transistor W/L wider and the resisitor bigger,
so Ictrl= Vcrtl/R.
2.A lineraized DAC can helpful,which is proposed
by M. Steyaert,which you can refer his paper
about synthesizer.
3.What you want is the total function of V-I and
f-I is linear,so f-V is linear.Just make the
total function linear, which ease your problem.

Click to expand...

in 1, I have made the nmos follower bigger, but the problem is resistor has the worse temperature characteristic than nmos follower. How to deal with the problem?
in 3, for VCO, most circuits can divide into two parts, V->I and I->f,because the VCO is controlled by current in essence. Can you provide the example that consider the “total function linear"?
Thanks a lot!
regards

Re: help me about VCO design

hanjiemy said:

u can use full differential V-I converter. it's linearity will be better.

Click to expand...

Can you provide more information or paper for me in details?
Thanks a lot!
Regards

help me about VCO design

ok! I will upload for u later!