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spiral inductor on pcb

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Jun 29, 2004
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pcb inductor

I have to design spiral inductor for my LNA, can any one help me designing spiral inductor on PCB?

How does the width, spacing and diameter effects the inductance and Q?

Substrate parameters are:

Rogers 4350
Dielectric Constanat (Er) = 3.48
Dielectric Thickness = 250 um
Loss Tangent = 0.004
Metal Thickness = 70 um
Conductivity = 5.8e7

Frequency band is 3-10 GHz.

thanks in advance.

If I want to design the inductor and Capacitor on straight TL then can some one forward me some formulas or recommend me some literature?

pcb inductance


All the parameters have an effect on both the inductance and the Q and usually changing one will also affect the other.

If you keep widden the width, you increase the inductance, at the same time this decreases the resistance, which improves your Q. As frequency increases this effect is deteriorated by the skin effect. The capacitnace between metal and ground increases. This lowers your resonant frequency.

The spacing between turns, controlles the resonant freqeuncy of the inductor. The less spacing, the more capacitance between the turns and the resonant frequency is lower. This is usually not wanted, but again it depends in what freqeuncy range it is applied.
The spacing is preferred to be low as this makes the inductor small.

I have designed some spirals myself and I found it usefull to leave out the inner turns of the spiral. This gave my a large increase in the Q.

The inductance is largely determined by the length of the spiral(N number of turns).

Hope this helped.


spiral inductor pcb

tyassin said:

I have designed some spirals myself and I found it usefull to leave out the inner turns of the spiral. This gave my a large increase in the Q.

Thanks for your reply. Can you please explain the above quoted sentence. How can I decide upon the number of turns? I have to design 1.2nH, 1.8 nH and 3.0 nH inductor.

One more thing, whats the difference in designing a series and parallel inductor?



The spiral inductor gets its "main" inductance from the physical properties i.e number of turns, width, spacing. But coupling between the different tracks of the spiral of cause influince each other. This is know as mutual coupling.
Very briefly: the sides having current flowing in same direction give positive mutual coupling, this is nice, this is why a spiral(which often is very small) can achieve relatively high inductance values. But the sides having current in opposite direction give rise to negative mutual inductance. As you can see, the inner turns of a spiral are very close and they contribute very little to the "main" inductance, because they are very short. But they introduce negative mutual inductance, because they are very close.

Because the inner turns contribute very little to the "main" inductance and the fact that the negative mutual inductance introduced by the inner can be larger than this, the inductance of the spiral remains almost the same.
Because you remove some of the metal from the spiral, you have lower losses and the Q goes up.

You can read an article by Greenhouse, I think its from the 60's. He introduces a calculation method for plannar spiral inductors with this mutual coupling. His apporach should be very precise, although somewhat difficult to program.

There are the classic formula by "Wheeler" and some formulas by other guys to design spirals, but they are only approximate. I found them as a good starting point and then used "that trial and error" :D.

I was not able to find any books with closed form exspressions that I could use.

But concerning your spirals, what process are you using?
What are the minimum spacing and width possible?

Puhh.....this was a long one.



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pcb inductor design

The frequency range is between 3-10 GHz. I am using GaAs pHEMT transistor.


inductor pcb

The formulas I refered too, take the number of turns (N), width(W) and spacing(S) as an argument and then gives you an inductance. We of cause would like to have it the other way around.

I do not know what CAD you are using, but I used the formulas to give me some direction and then I simply made it in ADS. The EM simulation take some time!! Then it was just minor adjustments.

My was used at a centerfrequency of 5.4 GHz and a BW=800 MHz. You should utilize the inductor where the Q is largest. You could have a problem because they should be used over a wide frequency range of 3-10 GHz.

You can check this link:
also there is an article at the bottom of the page you can look at.


inductors in pcb

The simulation software is @DS. i have desinged the inductor with Q of 30 but the variation in inductance is around 0.7 nH in this band. The parameters are:
W = 40 um
Turns = 2.5
S = 8 um

any suggestion how to improve it?

I want to know the difference between a serial inductor and a parallel inductor. Can some one put a light on it? Or there is no difference between the two?



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