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,
Vincent