[SI-LIST] Re: Routing of 12 GHz diff pairs
- From: "Erik Jacobs" <emj14@xxxxxxxxxxxx>
- To: <Giovanni.Guasti@xxxxxxxxxx>, <hannappe@xxxxxxxxxxxxxxxxxx>,<buck@xxxxxxx>
- Date: Wed, 2 Apr 2003 09:22:05 -0500
> Hi Buck,
> I think the best way to satisfy your customer is to draw the pair on two
> faced layer.
> But ... this method has many drawback:
> 1)it's more difficult to control vertical than horizontal distance between
> pairs(due to PCB layers compression). So you could achieve a pair with a
> wrong impedance.
> 2)in order to obtain the right impedance, you could require a bigger
> distance between layers
> I would prefer to draw the pair on the same layer, making a compromise: a
> max difference of 2.5mm (l/10) could be enough?
> > We are working with a customer that is using 12 GHz
> diff-pair signals.
> > The customer "requires" that the time of flight across the pair
> > "must" be equal at all points in time. Can any one shed any light
> > on how this can be done?
>
> The easiest way to achieve this is to use broadside coupled pairs,
> because there both conductors are identical and routing is therefore
> much esier.
I'm about to do some 10GHz routing with similar requirements. It seems that
if you are using coupled pairs and you made the physical length almost
identical, wouldn't the electrical length and electrical parameters be
pretty close? I mean, assuming the signals got onto the PCB the same way
and assuming that the material was relatively uniform and assuming you had
good ground planes (or ground wires running under the diff pair traces)...
Also don't you kinda need to assume that there's nothing around the traces
that will couple INTO them? I.e. if p side of diff couple has coupling to
some other line but N side has no such coupling? Even if it's only
capacitive/inductive coupling (non-crosstalk)
-Erik
Test Engineering
LeCroy
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