Transformer balun for high rejection unbalanced lattice filters
A transformer balun for high rejection unbalanced lattice filters includes two symmetrical coils separated by a shielding element. In a further exemplary aspect, the transformer may include a conductor, which may be a third coil that operates with a capacitor to form a resonant circuit that enhances mutual coupling. Using either of the exemplary transformers provides improved performance in the passband while concurrently providing out-of-band rejection at levels exceeding seventy decibels (70 dB).
1. A transformer comprising:
a first coil comprising an input;
a second coil inductively coupled to the first coil, the second coil comprising an output; and
a resonant circuit comprising a third coil and a third capacitor, the third coil inductively coupled to the first coil and the second coil but electrically isolated from the first coil.
2. The transformer of claim 1 , wherein the resonant circuit is coupled to ground.
3. The transformer of claim 1 , wherein the second coil is coupled to ground.
4. The transformer of claim 3 , wherein the resonant circuit is coupled to ground.
5. The transformer of claim 1 , wherein the first coil is symmetrical across an axis of symmetry.
6. The transformer of claim 1 , wherein the second coil is symmetrical across an axis of symmetry.
7. The transformer of claim 1 , further comprising a second capacitor, the second capacitor directly serially coupled to the second coil, wherein the second capacitor is coupled to ground.
8. The transformer of claim 1 , wherein the first coil and the second coil are co-planar in a first plane.
9. The transformer of claim 8 , wherein the resonant circuit is positioned in a second plane parallel to but different than the first plane.
10. The transformer of claim 8 , wherein the resonant circuit is positioned in the first plane.
11. The transformer of claim 1 , wherein the first coil is in a first plane and the second coil is in a second plane different from the first plane.
12. A filter system comprising:
a lattice filter comprising a lattice output; and
a transformer comprising:
a first coil comprising an input coupled to the lattice output;
a second coil inductively coupled to the first coil, the second coil comprising an output; and
a resonant circuit comprising a third coil and a third capacitor, the third coil inductively coupled to the first coil and the second coil but electrically isolated from the first coil.
13. The filter system of claim 12 , wherein the resonant circuit is coupled to ground.
14. A filter system comprising:
a lattice filter comprising a filter output; and
a transformer comprising:
a first coil comprising an input coupled to the filter output, the first coil having an axis of symmetry and being symmetrical across the axis of symmetry;
a second coil inductively coupled to the first coil, the second coil comprising an output, the second coil having a second axis of symmetry and being symmetrical across the second axis of symmetry; and
a shielding trace proximate the first coil and the second coil, the shielding trace configured to reduce cross-capacitance between the first coil and the second coil, wherein the shielding trace is coupled to ground.
15. The filter system of claim 14 , wherein:
the first coil and the second coil are co-planar in a first plane; and
the shielding trace comprises a third conductor positioned between the first coil and the second coil in the first plane.
16. The filter system of claim 14 , wherein:
the first coil and the second coil are co-planar in a first plane; and
the shielding trace comprises a third conductor positioned in a second plane parallel to but different than the first plane.
17. The filter system of claim 16 , wherein the shielding trace further comprises a fourth conductor positioned in a third plane parallel to but different than the first and second planes.
18. The filter system of claim 14 , wherein the output comprises a first output and a second output and the second output is coupled to ground.
19. The filter system of claim 14 , wherein the shielding trace comprises a third conductor and the third conductor defines a cut therein.
20. The filter system of claim 14 , wherein the first coil is positioned in a first plane and the second coil is positioned in a second plane different than the first plane.