[SI-LIST] Differential trace Characteristic Impedance
- From: "Steve Rogers" <SRogers@xxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 21 Jan 2004 12:53:41 -0000
I need to run USB signals around a board using differential traces. I =
found the following equations for Diff Zo from the ultracad paper by =
Doug Brooks:
Zdiff =3D 2*Zo[1- 0.48*e^(0.96*(S/H)]
Zo =3D 60*Ln(5.98*H/(0.8W + T)](Er +1.41)^0.5
My trace to ground spacing (H) =3D 0.5mm (If possible).=20
Problem
USB 1.1 calls for 90 ohm diff Zo (+/- 15%). Using the equations above I =
can get 90 ohm diff Zo with a number of trace spacings and thickness =
combinations. Of course the Zo for the isolated trace also changes, I am =
not sure what this really wants to or should be. Its not defined in the =
USB1.1 SPEC. So do I put the two traces really close together or space =
them ? Any suggestions on this would be helpful. I have a feeling that =
the Zo equation above may not be good enough as it doesnt account for =
the presence of another trace ?
HELP
Thanks in advance=20
Steve Rogers B.Eng (Hons) C.Eng IEE
RF Design Engineer
Micromill Electronics Limited
Leydene House
Waterberry Drive=20
Waterlooville Hampshire
PO7 7XX
Tel: +44 (0) 23 9236 6600
Fax: +44 (0) 23 9236 6673
Registered No. 1456922 (England). =20
Registered Office Brook Road
Wimborne, Dorset BH21 2BJ
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