Setting the impedance of signal traces of a circuit board using a reference trace

1. A circuit board comprising:

a dielectric layer;

a first contact finger and a second contact finger that are on an edge connector of the circuit board, the first and second contact fingers being on a first surface of the dielectric layer and configured to carry a differential signal;

a reference plane that is on a second surface of the dielectric layer; and

a reference trace that is within the dielectric layer, the reference trace being directly under and between the first and second contact fingers, the reference trace being configured to set a differential impedance of the first and second contact fingers,

wherein the reference plane, the reference trace, the first contact finger, and the second contact finger each comprises a metal.

2. The circuit board of claim 1 , wherein the reference plane is between the dielectric layer and another layer of the circuit board.

3. The circuit board of claim 1 , wherein the differential impedance of the first and second contact fingers is set to about 85 ohms.

4. The circuit board of claim 1 , wherein the reference trace has a width that is narrower than a spanning distance between outer side perimeters of the first contact finger and the second contact finger.

5. The circuit board of claim 1 , wherein the reference traces comprises copper.

6. The circuit board of claim 1 , wherein the edge connector includes a notch.

7. The circuit board of claim 1 , wherein the reference trace has a first portion that is in parallel with the first and second contact fingers and a second portion that is perpendicular to the first portion.

8. The circuit board of claim 1 , wherein the first and second contact fingers comprise gold.