Marchand-type distributed balun

1. A mode-switching transformer comprising a first conductive line between two common-mode access terminals and a second conductive line between two differential-mode access terminals, said lines being coupled and sized according to a central operating frequency of the transformer, wherein each line is formed of two stacked spirals in two different metallization levels, each spiral of a line being interdigited with a spiral of the other line;

wherein a third metallization level is used to connect outside of the structure a central contact of a winding of one of the conductive lines.

2. A mode-switching transformer comprising a first conductive line between two common-mode access terminals and a second conductive line between two differential-mode access terminals, said lines being coupled and sized according to a central operating frequency of the transformer, wherein each line is formed of two stacked spirals in two different metallization levels, each spiral of a line being interdigited with a spiral of the other line;

wherein a third metallization level is used to connect outside of the structure a central contact of the winding, and

wherein all points of access to the transformer are formed in a same metallization level, the number of vias being limited to no more than five for the entire structure.

3. The transformer of claim 1 , wherein the spirals are sized based on an inductance value desired for a non-stacked structure, knowing that the value increase is proportional to the square of the number of turns of the stacked structure.

4. The transformer of claim 2 , wherein the spirals are sized based on an inductance value desired for a non-stacked structure, knowing that the value increase is proportional to the square of the number of turns of the stacked structure.

5. A mode-switching transformer comprising:

two common-mode access terminals;

a first conductive line connecting the two common-mode access terminals;

two differential-mode access terminals;

a second conductive line connecting the two differential-mode access terminals, the first and second conductive lines being coupled to one another;

the first conductive line having a first stacked spiral in a first metallization layer and a second stacked spiral in a second, different metallization layer;

the second conductive line having a third stacked spiral in the second metallization layer, the third stacked spiral being interdigited with the second stacked spiral;

the second conductive line having a fourth stacked spiral in the first metallization layer, the fourth stacked spiral being interdigited with the first stacked spiral; and

a central contact disposed in a center of a stacked spiral of one of the first and second conductive lines, the central contact being configured to be connected to a component outside of the transformer, and the central contact being formed in a third metallization level that is different from the first and second metallization levels.

6. The transformer of claim 5 , wherein the central contact comprises a central contact of the first conductive line.

7. The transformer of claim 5 , wherein the central contact comprises a central contact of the second conductive line.

8. The transformer of claim 5 , wherein all points of access to the transformer are in the third metallization level.

9. The transformer of claim 5 , wherein the transformer comprises no more than five vias.

10. The transformer of claim 8 , wherein the transformer comprises no more that five vias.

11. The transformer of claim 8 , wherein the transformer comprises exactly five vias.

12. The transformer of claim 5 , wherein the first and second conductive lines have lengths that are sized to provide a central frequency of a desired passband for the transformer.