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The strange structure of inductor of LC VCO

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walkingsun

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lc-vco capacitor bank

What's the 'green' structure in the picture. It has any advantage?
Thank you!
 

Mazz

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It is only a capacitor to tune the resonator.
Inductor is not shown and is done using bondwires. As there is usually a certain level of inaccuracy in the bondings inductance, designers add this multifinger cap to adjust the resonant frequency (it requires new top mask to adjust it).

I hope it can help

Mazz
 

    walkingsun

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walkingsun

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In order to tuning resonate frequency.
I got it~
Thank you, Mazz!
 

BigBoss

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But on silicon substrates fringe capacitors are rarely used because of very low capacitance values.In this photo, with this fringe capacitor ( depending of course on tech. ) will show very small amount of capacitance value..around few fF maybe less. With this value, any bonding inductance can not be in a resonating witha reasonable frequency.
I think it's a ESD protection diode.. not a fringe cap. nor square spiral inductor..If the photo was coloured, we can say more precisely..
It looks a little bit strange..
 

Mazz

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BigBoss

In the showed plot, the inductor is the bonding.

usually this structure is used in advance of classical varactor tuning+ cap banks for VCO sub-bands.

Imagine you need to go to a fast production run and you have an high freq VCO (5 GHz).
With 1 nH inductor, just as an example, and a total switched cap bank of 1 pF, you got the correct freq.
How to deal with inaccuracy of bonding/unaccounted parasitics?
Adding such a module (for example 50 fF total), you can adjust in a couple of weeks (only last metal of a properly stopped process) 2.5% the VCO freq.

Don't tell YOUR Big Boss who gave you this hint... (joking of course).

Walkingsun, could you please attach the complete IEEE paper?

Mazz
 

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