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summi Community Member Offline Posts: 38	LC tank Q factor Nov 19^th, 2011, 1:16pm Dear forum, for an LC tank always Q dominated by Inductor Q, because it is lower than varactor Q. Corect me if any thing wrong. And is this domination always true or any exceptions? Br, Summi.
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rfidea Senior Member Offline Posts: 159 Europe	Re: LC tank Q factor Reply #1 - Nov 19^th, 2011, 2:02pm That is usually true but not always. If you have an low loss air coil inductor and a lossy varactor it is maybe the varactor that is dominating the Q.
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: LC tank Q factor Reply #2 - Nov 19^th, 2011, 5:58pm unique to the components available. Low Q inductors are common in semiconductors, but there are exceptions. (special dielectrics, and low resistance metal structures) Need to consider all the elements used and what their non ideal characteristics are.
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: LC tank Q factor Reply #3 - Nov 20^th, 2011, 3:15am hello, I donno about other meterial passives like dielctric and loss air coil. But in IC level passives IND Q increases with frequency and Varactor Q decreases with Frequency, so for a given area and technology there is frequency below that inductor Q dominates and above varactor Q dominates. Thanks, Raj.
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: LC tank Q factor Reply #4 - Nov 21^st, 2011, 2:14pm generally true, yes, but if you get into certain SOI stuff (thinking sapphire here) there are some situations where the parasitics are very different.
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: LC tank Q factor Reply #5 - Nov 22^nd, 2011, 2:04am hello looselectron, i didn't get u..Normally paracitic caps Q will be higher (infact inf), so if you have paracitics across the tank so overal Q will be dominated i guess. Sorry please explain if you find any mistake Thanks, Raj.
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singh

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Re: LC tank Q factor
Reply #6 - Nov 22^nd, 2011, 8:56am

summi wrote on Nov 19^th, 2011, 1:16pm:

Dear forum,
for an LC tank always Q dominated by Inductor Q, because it is lower than varactor Q. Corect me if any thing wrong. And is this domination always true or any exceptions?

Br,
Summi.

if u use offchip inductor, then its Q will be higher than varactor..generally to minimize losses, off chip inductor is preferred

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: LC tank Q factor Reply #7 - Nov 22^nd, 2011, 10:27am hello singh, you are correct but at low frequency, when you go to high frequency or IC level Q will be increased. Thanks, Raj.
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loose-electron

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Re: LC tank Q factor
Reply #8 - Nov 29^th, 2011, 3:59pm

raja.cedt wrote on Nov 22^nd, 2011, 2:04am:

hello looselectron,
i didn't get u..Normally paracitic caps Q will be higher (infact inf), so if you have paracitics across the tank so overal Q will be dominated i guess.

Sorry please explain if you find any mistake

Thanks,
Raj.

Raj - the message I am trying to give here is that the conditions are unique for the foundry process.

Parasitics are different, resistance of metal layers are different. Both affect inductor Q in a big way.

So, the question has to be looked at within the limitations of a particular foundry process.

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Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical

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