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Design >> Analog Design >> Enhance the Speed of LC-VCO
https://designers-guide.org/forum/YaBB.pl?num=1166706266

Message started by neoflash on Dec 21^st, 2006, 5:04am

Title: Enhance the Speed of LC-VCO
Post by neoflash on Dec 21^st, 2006, 5:04am

The simplest way to increase the speed of LC-VCO is to reduce the inductance of the tank.

Thus, in theory, we can oscillate really fast.

However, I guess we trade something off for faster speed. What is the price? What is the criterion in choosing inductor value?

Thanks,
Neoflash

Title: Re: Enhance the Speed of LC-VCO
Post by topquark on Dec 21^st, 2006, 6:13am

First off, you can shrink L so much as you increase C at a given frequency f_RES.
Choice of integrated L value is primarily affected by 2 criteria:
1) Effective tank conductance G_TK varies inversely as L
So, larger L=> larger Q_L and lower G_TK

2) phase noise L(Δω) varies directly as R_L, the series resistance of the inductor
larger L=> larger R_L and poorer L

It follows, largerL=> lower power consumption, lower f_RES and f_Q-PEAK, the freq at which Q peaks.
So, we choose the largest L having peak Qclose to the operating frequency