Using a portion of differential signal line to provide an embedded common mode filter

1. A circuit board configured to have an integrated circuit mounted thereon and comprising at least one differential signal line to be connected to the integrated circuit, the differential signal line being provided with a first portion having a higher common mode impedance than another portion of the differential signal line, the circuit board further comprising an impedance reference plane that has a hollow in a region of the first portion to provide that portion with the higher common mode impedance, the portion of higher common mode impedance being configured to provide a common mode noise filter.

2. The circuit board of claim 1 , wherein the hollow extends for approximately 10 mm along the differential signal line.

3. The circuit board of claim 1 , wherein the hollow is formed at a distance along the differential signal line from a contact to which the integrated circuit is connected of at least half the wavelength of the lowest frequency component to be filtered.

4. The circuit board of claim 1 , wherein the differential signal line is configured to provide a substantially constant differential impedance along its length.

5. The circuit board of claim 1 , wherein the width of each portion of the differential signal line is chosen to provide a substantially constant differential impedance per unit length along the differential signal line.

6. The circuit board of claim 1 , comprising a plurality of differential signal lines.

7. The circuit board of claim 1 , comprising broadside coupled differential signal lines.

8. A circuit board having at least one integrated circuit mounted thereon, the circuit board further comprising at least one differential signal line connected to the integrated circuit, the differential signal line being provided with a first portion having a higher common mode impedance than another portion of the differential signal line, the circuit board further comprising an impedance reference plane that has a hollow in a region of the first portion to provide that portion with the higher common mode impedance, the portion of higher common mode impedance being configured to provide a common mode noise filter.

9. A method of filtering clock noise from an integrated circuit, the method comprising providing at least one differential signal line connected to the integrated circuit with an embedded common mode filter, wherein the differential signal line is provided with a portion having a higher common mode impedance than another portion of the differential signal line to form the embedded common mode filter; and

providing a hollow in an impedance reference plane in the region of the differential signal line portion having the higher common mode impedance to provide that portion of the differential signal line with the higher common mode impedance.

10. The method of claim 9 , comprising providing a distance that the hollow extends along the differential filter line according to a desired filter characteristic.

11. The method of claim 9 , wherein the hollow extends for approximately 10 mm along the differential signal line.

12. The method of claim 9 , comprising providing the hollow at a distance along the differential signal line from a pin of the integrated circuit to which the differential signal line is connected according to the lowest frequency to be filtered.

13. The method of claim 9 , comprising providing the differential signal line with a substantially constant differential impedance along its length.

14. The method of claim 9 , comprising configuring a width of each portion of the differential signal line to provide a substantially constant differential impedance per unit length along the differential signal line.