[SI-LIST] Re: Differential trace Characteristic Impedance
- From: "Steve Rogers" <SRogers@xxxxxxxxxxxxx>
- To: "Vincenzo Kreft-Kerekes" <vincenzo@xxxxxxxxxxxxxx>
- Date: Thu, 22 Jan 2004 16:12:37 -0000
Vincenzo,
Unfortunately the best case scenario is running USB diff tracks a =
distance of about 200mm. If I could do I would reduce the lengths.=20
I want to avoid the high speed USB kit as I am running the USB kit in an =
environment with some sensitive radio equipment. I dont want to jam my =
radio kit!
Any idea on the track dimensions etc ? =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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-----Original Message-----
From: Vincenzo Kreft-Kerekes [mailto:vincenzo@xxxxxxxxxxxxxx]
Sent: Thursday, January 22, 2004 4:07 PM
To: Steve Rogers
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: Differential trace Characteristic Impedance
Steve,
Quick question diverging from differential trace impedance: What =
distance do
these USB traces need to cover? Does running them "around the board" =
mean
literally around a bunch of components. What's the geometry here? Do you
have the board space to place a PHY or other receiving device direcly =
next
to the connector and worry about getting an 8 bit bus at some 20 Mhz =
around
the board instead of dealing with differential impendances at 480Mhz?
Apple's new server modules for instance have I believe two Firewire 800
(1Gb/s, .5ns edge) ports spaced far apart and one PHY on each port to =
avoid
routing the differential traces for long distances.
Also, when I see USB 1.1 mentioned, are you sure you don't want to use a
high-speed PHY with USB 2.0 instead? They come in much smaller packages =
than
USB 1.1 PHYs and your product would be as fast as USB can be for some =
time
to come.
Best,
Vincenzo
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Steve Rogers
Sent: Thursday, January 22, 2004 5:55 AM
To: Doug Brooks; Fred Balistreri; steve weir; mediwheel_js
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Differential trace Characteristic Impedance
First thanks for your help guys.
Taking all you have said into consideration it seems like I either need =
=3D
to use a field solver, the Ultra-cad program or guess!
While I decide on the way to go consider the following:-
1. I could space my tracks far enough so that I could use the simple =3D
equations from national semiconductor. This is great but how far apart =
=3D
do the tracks need to be in order that they have no affect on each =3D
other. Next, I would think that this approach is wasteful of board =3D
surface area given all the unused space between the diff tracks. This =
=3D
said there are probably some benefits of doing things this way. Theres =
=3D
bound to be somthing good about having a very small contribution of diff =
=3D
Zo made up from coupling between tracks?
2. Going the other way I can try to claw back the lost space between the =
=3D
tracks. Once I do this the diff Zo begins to drop from twice the =3D
isolated single track Zo value. In order to compensate I then just need =
=3D
to reduce the thickness of the tracks. This is nice as within sensible =
=3D
limits I end up with an even more narrow overall structure =3D
(trace-space-trace).
Of course The board house must be able to manufacture the trace and gap. =
=3D
With this approach We have the opposite situation with a large =3D
contribution of diff Zo coming from fringe field or coupling between =3D
traces (whatever you want to call it). Is this a clever thing to do? I =
=3D
would imagine that the tolerance with this approach is going to be a lot =
=3D
worse.
Any comments? =3D20
Steve Rogers B.Eng (Hons) C.Eng IEE
RF Design Engineer
Micromill Electronics Limited
Leydene House
Waterberry Drive=3D20
Waterlooville Hampshire
PO7 7XX
Tel: +44 (0) 23 9236 6600
Fax: +44 (0) 23 9236 6673
Registered No. 1456922 (England). =3D20
Registered Office Brook Road
Wimborne, Dorset BH21 2BJ
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