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Design >> Mixed-Signal Design >> LC tank Q factor
https://designers-guide.org/forum/YaBB.pl?num=1321737415

Message started by summi on Nov 19^th, 2011, 1:16pm

Title: LC tank Q factor
Post by summi 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.

Title: Re: LC tank Q factor
Post by rfidea on 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.

Title: Re: LC tank Q factor
Post by loose-electron on 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.

Title: Re: LC tank Q factor
Post by raja.cedt on 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.

Title: Re: LC tank Q factor
Post by loose-electron on 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.

Title: Re: LC tank Q factor
Post by raja.cedt 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.

Title: Re: LC tank Q factor
Post by singh on 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

Title: Re: LC tank Q factor
Post by raja.cedt on 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.

Title: Re: LC tank Q factor
Post by loose-electron on Nov 29^th, 2011, 3:59pm

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

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.