Why Phase Noise in CMOS LCtank VCO is better than the Ring?

Dear all,

Could anyone explain to me that why the phase noise in the CMOS LC-Tank VCO is better than the CMOS Ring Oscillator?

I have this explanataion. I don't know is correct or not. I hope any one could correct me if I am wrong.

Since in the Ring Oscillator, the oscillating frequency is due to the delay time of each NOT gate. However, the dealy time of the gate is highly depends on the mobility of the transistor and the temperature. Therefore, the delay time of time to time is different and make the jitter is always appeared. So, the phase noise in the ring oscillator is worse.

However, the LC-tank VCO has not this big problem. Therefore, the phase noise is better.

Am I this explanation correct or not?

Best Regards,

wccheng

Re: Why Phase Noise in CMOS LCtank VCO is better than the Ri

The quality factor of the ring osc is about 1 (Q=the ratio of the energy stored to the energy dissipated per cycle), because in one cycle period, the energy component (the capacitance) is charged in one half cycle and discharged in the other half cycle, and the energy stored and dissipated on the capacitance is almost equal, and then the Q is about 1.
For LC osc, the Q is higher than 1.
And from the classical Leeson's theory, higher Q means better phase noise performance.

Re: Why Phase Noise in CMOS LCtank VCO is better than the Ri

Ring oscillator is a wideband oscillator while the LC tank is a narrow band one. So the noise couppled form enviornment to the oscillator is lager than that of LC tank oscillator. Does my comments make sense?

Re: Why Phase Noise in CMOS LCtank VCO is better than the Ri

check section V-B in this paper and read the answer...

1.The quality factor of the ring osc is about 1 For LC osc, the Q is higher than 1. And from the classical Leeson's theory, higher Q means better phase noise performance.
2.Ring oscillator is a wideband oscillator while the LC tank is a narrow band one. So the noise couppled form enviornment to the oscillator is lager than that of LC tank oscillator.
3.Since in the Ring Oscillator, the oscillating frequency is due to the delay time of each NOT gate. However, the dealy time of the gate is highly depends on the mobility of the transistor and the temperature. Therefore, the delay time of time to time is different and make the jitter is always appeared. So, the phase noise in the ring oscillator is worse.
4.in the ring oscillator you have some noise source such as transistors and in LC you have just one transistor as noise source.