Setting the impedance of signal traces of a circuit board using a reference trace
A circuit board has an edge connector with signal traces. The signal traces are formed on a dielectric layer of the circuit board. A reference trace is formed within the dielectric layer or on another surface of the dielectric layer. Parameters of the reference trace are adjusted to set an impedance of a single-ended signal trace or a differential impedance of two adjacent signal traces.
1. A circuit board comprising:
a dielectric layer;
an edge connector disposed on a first surface of the dielectric layer, the edge connector comprising a plurality of contact fingers, a first contact finger and a second contact finger of the plurality of contact fingers being adjacent and configured to carry a differential signal; and
a reference trace within the dielectric layer, the reference trace being disposed directly under and between the first and second contact fingers, wherein the reference trace sets a differential impedance of the first and second contact fingers.
2. The circuit board of claim 1 , wherein the differential impedance of the first and second contact fingers is set to about 85 ohms.
3. The circuit board of claim 1 , wherein the reference trace has a width that is narrower than a spanning distance between outer perimeters of the first and second contact fingers.
4. The circuit board of claim 1 , further comprising a notch on the edge connector.
5. The circuit board of claim 1 , wherein the reference trace comprises copper.
6. The circuit board of claim 1 , further comprising a reference plane disposed on a second surface of the dielectric layer.
7. The circuit board of claim 1 , wherein the reference trace has a longitudinal portion that is in parallel with lengths of the first and second contact fingers.
8. The circuit board of claim 7 , wherein the reference trace has a T-shape with another portion that is perpendicular to the longitudinal portion.