vco lc+q

Hi guys,

I got a little confused in determine the parameters in LC VCO. Normally, people make gmRtank>2. I don't have the accurate varactor model, so I assume the Q value is twice of the inductor. So, I my design, I make gmRinductor>3.

I got inductor's PI model from ASITIC and the Q value is something around 15 @ 5GHz. I do not think it is reliable because I didn't take the eddy current into consideration. So again, I assumed Qinductor is 10. Thus, I meet the following problem:

1. When I calculate the shunt resistance of the inductor, I use wLQ or Rs*Q*Q ? (the Rs is around 1.5 ohm), if I Q=15, wLQ=Rs*Q*Q. However, I assume Q can not reach such a high value. So what should I do?

2. Assume the Rinductor is 300 ohm, the gm should be greater than 0.01 according to gmRindctor>3. So, how can I determine the W/L of the NMOS? I use 1V supply. Can I just apply 1V voltage to the drain and gate terminals and evaluate the gm with 2Id/(Vgs-Vth) ? Please help. THX

You should first determine which structure you want to use.Then the gm could be calculated by the operating point.

Hi,

In your example, How we calculate the gmRtank>2?

Thanks!