How on earth do I route a differential pair?

I think you must route one wire at a time...slow & somewhat laborous, but

possible.  Pehaphas one on  toplayer & the other on a layer below.

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On 12/19/2011 4:18 PM, Alex Whittemore wrote:

I've searched high and low in docs and the forums, but I still can't figure out how to do it. I have a net named USB_M and a net named USB_P, both of class 0. The best I've figured out how to do is put a meander in one side, and even still that doesn't work because when I try to meander close-together traces, one INVARIABLY tries to overlap the other.

 

So how am I SUPPOSED to route a diff pair?

 

Hi Alex,

It should be USB_N not USB_M, make sure to use the ROUTE command and not

the wire command otherwise it won't work.

hth,

Jorge Garcia

Ok, that's super non-standard, but at least it works. Everyone I've ever read schematics from in the circuit design business uses P (plus) and M (minus). I'd vote that future releases support any obvious pairing, P/M, P/N, +/-.

Better question: What do I do about the differing lengths? I can't add meanders since they apply to both traces. This doesn't really fix much. And finally, do I get to set transmission line parameters like impedance (assuming FR-4) or at least spacing?

On 12/22/2011 1:31 PM, Alex Whittemore wrote:

Ok, that's super non-standard, but at least it works. EveryoneI've ever read schematics from in the circuit design business uses P

(plus) and M (minus). I'd vote that future releases support any obvious

pairing, P/M, P/N, +/-.

 

Better question: What do I do about the differing lengths? I can't add meanders since they apply to both traces. This doesn't really fix much. And finally, do I get to set transmission line parameters like impedance (assuming FR-4) or at least spacing?

 

Hi Alex,

I don't know, P(ositive) and N(egative) make sense to me, but not a bad

suggestion.

You have to use the meanders command but getting it to work require some

finesse. Right now it doesn't support the ability to specify a specific

impedance you want to route to, however there are tools available the

will calculate trace width, length and separation to achieve a specific

impedance(Saturn PCB's PCB toolkit comes to mind).

Let's say you have two long segments of the differential route and you

decide that this is where you want to add the meanders to equalize the

length. Activate the meanders command and left click on the shorter of

the two traces, where you click is important it's best to click on the

rightmost(leftmost point) instead of the middle of the trace. So

assuming you click on the rightmost point move your mouse towards the

left you should see that if you move far enough away EAGLE will start to

produce the meanders.

In DRC misc. tab you can setup the Gap Factor and the tolerance on the

lengths.See HELP MEANDER for more options on the meander command. It

takes a little bit of play but you'll get it.

hth,

Jorge Garcia