Hi Gurus,

A high shunt impedance is needed at low-frequency (~500MHz), 30nH inductance is approx. ok. However, this inductor is huge on silicon(SiGe Bicmos technology), its fres is about 2GHz. Wanna to use an inductor which has fres at ~500MHz, then the impedance at fres=500MHz is high enough. But I think the inductor with fres=500MHz is not possible to be made smaller than the original one: L=30nH, fres=2GHz. Because, fres=1/sqrt(LC), for lower fres, L needs to be large, maybe change to lower metal layers can help increase C, but fres will not be reduced too much.
Am I right? or there are other methods to make a relatively small inductor but with very low fres?

Your help is highly appreciated.

An rf-design beginner.

Hi Aaron,

Thank you for your reply.

The 30nH inductor is actually used as an inductive shunt for ESD protection. Adding a parallel cap will this cause possible oscillation problem?
And using inductor at resonance is a very narrow band solution for RF performance, right?

Kind regards
rf_man

yes, I considered the ESD diodes. Due to the parasitic diodes to substrate, the stacked diodes solution does not work.

Could you please indicate whether there is risk of oscillation due to adding parallel C to form an LC-tank?

Many thanks.

rf_man

Hi Aaron,

It's a front-end IC.

Suppose that use a relatively small inductor together with a parallel C to tune the resonance frequency to 500MHz. Then the risk of oscillation is that it may oscillate at 500MHz? If just use the 30nH inductor without adding parallel C, then it might oscillate at a higher frequency (determined by 30nH and the parasitic C to the ground)?

Now I am confused with the self-resonance frequency, e.g. an inductor 's fres=2GHz, then it means that it will always oscillate at 2GHz? Below 2GHz, it functions as an inductor; above 2GHz, it functions as an capacitor. But many inductors are used in a circuit which operates far above the fres of the inductor without oscillation at fres of the inductor, right? Please help me to understand this.

Kind regards,

Hi,


apart from the above request...

resonance does not imply oscillation.  First, when we say oscillate, we mean oscillate forever or until the oscillation is disrupted. It you are talking about an oscillation that gradually dies down, then we usually call it "ringing". In order for a resonator to oscillate, the same amount of energy that is dissipated in the parasitic resistance must be added back, usually by positive feedback (unwanted in your case). In fact, there is always a risk of oscillation even without an inductor. But I would say that inductors increase the risk.

have I answered your question?


Aaron