# about the calculation of LC VCO

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#### Ding Yan

##### Newbie level 4
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

#### mengcy

##### Member level 5
You should first determine which structure you want to use.Then the gm could be calculated by the operating point.

#### Chest

##### Member level 3
Hi,

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

Thanks!

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