Circuit board having traces with distinct transmission impedances

1. An optoelectronic assembly comprising:

an optoelectronic device

a circuit board including:

first and second signal traces in electrical communication with the optoelectronic device;

a plurality of dielectric layers mounted below the first and second signal traces; and

a plurality of ground planes, each ground plane alternating with a dielectric layer of the plurality of dielectric layers;

wherein, for each ground plane of a subset of the plurality of ground planes, no portion of the ground plane extends substantially below the first signal trace; and

wherein a width of the first signal trace and a width of the second signal trace are substantially similar, but an impedance of the first signal trace and an impedance of the second signal trace are substantially different.

2. The optoelectronic assembly of claim 1 , wherein a height and composition of the first signal trace are substantially similar to a height and a composition of the second signal trace.

3. The optoelectronic assembly of claim 1 , wherein the subset of ground planes, as well as the portion of each ground plane in the subset, govern the impedance of the first signal trace.

4. An optoelectronic assembly, comprising:

a circuit board including: a plurality of signal traces, a first ground plane, a second ground plane, a first dielectric layer disposed between the signal traces and the first ground plane, and a second dielectric layer disposed between the first ground plane and the second ground plane;

the second ground plane having a first portion and a second portion, wherein the first ground plane overlays the first portion of the second ground plane, but does not overlay the second portion of the second ground plane;

a first signal trace of the plurality of signal traces configured to substantially overlay the first ground plane, and having a first transmission impedance that is substantially determined by a thickness of the first dielectric layer; and

a second signal trace of the plurality of signal traces configured to substantially overlay the second portion of the second ground plane, and having a second transmission impedance that is substantially determined by a combined thickness of the first and second dielectric layers.

5. The optoelectronic assembly of claim 4 , wherein the first signal trace and second signal trace have substantially similar composition, height and width.

6. The optoelectronic assembly of claim 4 , wherein the first transmission impedance is less than the second transmission impedance.

7. The optoelectronic assembly of claim 6 , wherein the first transmission impedance is approximately half the second transmission impedance.

8. The optoelectronic assembly of claim 4 , wherein a width of the second portion of the second ground plane is no less than a width of the second signal trace plus two times the combined thickness of tho first and second dielectric layers multiplied by three.

9. The optoelectronic assembly of claim 4 , wherein enough of the first ground plane does not overlay the second ground plane to ensure that the first ground plane does not significantly affect the second transmission impedance of the second signal trace.

10. The optoelectronic assembly of claim 4 , further the circuit board further comprises:

a third signal trace;

third ground plane; and

third dielectric layer disposed between the second ground plane and the third ground plane;

wherein the third ground plane has a first portion and a second portion, and wherein the second ground plane overlays the first portion of the third ground plane, but does not overlay the second portion of the third ground plane; and

wherein a third transmission impedance of the third signal trace is substantially determined by a combined thickness of the first, second, and third dielectric layers.