How to increase the BW of the spiral inductor?

I am designing a spiral inductor, but its -10dB bandwidth is too narrow, how to make it wider?
Many thanks.

Hi,
Reduce the length of the indctor or try to reduce the inductance.

I think you talk about Q factor because there isn't such "Bandwidth" definition for an inductor in the literature.
Reduce Q factor by connecting a resistor in parallel or increase series resistance of the coil by using narrower strip width or decrease overall diameter.

Depends on the real purpose. For instance, conical coils are best as chokes up to ~50 GHz

tony_lth said:

I am designing a spiral inductor, but its -10dB bandwidth is too narrow, how to make it wider?
Many thanks.

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Decrease Q factor and by the way play with sqrt L/C.

You have to reduce the Q factor of the inductor:
- Inductor Q-factor is proportional with inductance.
- Width-to-spacing ratio and the outside diameter affect the Q of the spiral inductor.
- The Q-factor decreases with a decrease in the outside diameter.
- The Q of an inductor can be decreased by using a thin copper metal.
- Rectangular inductors have lower Q than circular inductors.

Thank you very much for so many ideas.

If I want to shifter the center freq to lower, what should I do?

tony_lth said:

If I want to shifter the center freq to lower, what should I do?

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Add several turns and make the coil tighter. More turns makes inductance larger, tighter turns increase the capacitance. Both shift the resonance frequency down.

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At frequencies lower than ~500 MHz you can try to insert a ferrite core but it often increases the loss (wider BW)

If I understand it right you do not want to increase the bandwidth (an inductor does have a bandwidth in theory, but because of paracitics it has a selfresonance frequency and you can shift that by building it in certain ways) A LC combination has a bandwidth. This you can increase by adding resistance but this is also adding loss.

If I understand it right You need to increase the selfinductance. This can be done by increasing the number of turns or a larger diameter or core material etc. Download Analog SEEKrets from L Green, it is a free Download and you find allmost everything you want to know. Including completer formulas for coils and not only the simplified Wheekler. To get a better view on the different forms of resonant and the math behind it AC Theory from D Knight is a good and also free download.

What frequency are you talking about en what sort of coil ?

A ferite core can also increase the Q because you use less turns and can get less distributed capacitance. But core material can add loss and you can get things like saturation. I made a 500 kHz HAM transmitter and my m,atching network with core material added to much loss. Air gave the best result but in a 10 MHz transmitter I had better results by adding a core.

Actually I used two layers, so it has bandwidth. Thanks.

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In CST, does it has any way to automatically setup spiral indcutor? I think so, but I don't know how to do.
I have to input every corner's axis position in CST now, and it's really bored.

tony_lth said:

Actually I used two layers, so it has bandwidth. Thanks.

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In CST, does it has any way to automatically setup spiral indcutor? I think so, but I don't know how to do.

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Yes it`s possible.