planar transformer cup

Does anyone have any info on how to design an RF planar transformer with 2 center tapped windings operation at about 100 MHz

Any help greatly appreciated



Timelord

You do two spiral inductors on adjacent PC card planes. It is best if they are offset so that the mutual capacitance is less. To make the center tap on each, you actually do four spiral inductors on 4 planes and hook them up in pairs in series. This is less than ideal being air core because the mutual inductance is less than a ferrite core version, but it is the best you can do.

You can go one further with putting a hole in the center and a few holes around the outside circumfrence of the coils and put two sort of ferrite cup cores over the front and back of the board. This is relatively expensive.

The smaller mutual inductance will make the voltage and impedance ratios different than an ideal transformer.

You can try Magnetics site: http://www.mag-inc.com/library.asp. There are a lot of references concerning magnetics designs, including planar transformers and inductors.

Ebrio31

Many thanks to you both. I think I have an idea how to start now.


Timelord

Here is some reference materials. Hope that it is useful for you!

Rgs

Modelling and Characteristics of On-Chip Transformer

Thanks Rayengine. Much appreciated. One question though.
In the center tapped model. Do you think the tap would be at the junction
of the inner and outer windings?


Rgs

Timelord