VCO starting factor of three alpha/safety

[mazelk]

VCO starting constrain

According with some reported VCO designs, active transconductance must overcome to tank transconductance, at least, in a factor of three (that they call "alpha" factor, or safety factor)

Why this amount?

I guess that it's not completelly true. As you increase "alpha", you increase transistor geometry (for same bias current) and drain capacitance (I mean about a complementary LC VCO). This reduce LC tank inyected current, and voltage swing. This increase phase noise.

Regarding those assumptions, which "safety factor " should I take to overcome tank loses?

Is it polite to consider that increasing "safety factor" could lead to drawback ??

 

[pi331133]

VCO starting constrain

I think it is necessary to keep the safety factor, to increase safety factor, you can increase W/L, if increase W can cause the problem you said, then you can reduce L. Also LC tank is a key in VCO. The Q has effect on safety factor also. If the safety factor is low, the simulation may hard to convergy

 

[mazelk]

Re: VCO starting constrain

Is the VCO starup constrain related with synthesizer control signal bandwidth ??

I mean, increasing "alpha" LC tank poles are rigth shifted, decreasing damping and decreasing "Tao" for exponential envelope.