[SI-LIST] Re: [SI-LIST]Loosely Coupled vs Tightly Coupled
- From: Jonathan Riley <jonathan.lloyd.riley@xxxxxxxxx>
- To: aaditya.kandibanda@xxxxxxxxx
- Date: Tue, 25 Nov 2014 23:12:31 +0000
Most of the time loose coupled will be better. It is best to start by
saying that the assumption here is that there is a reference plane very
close by both signals in the pair. If this is true, its influence on the
impedance dominates all the others. For constant differential impedance
bringing the traces closer together also means that they must become
thinner - so your losses rise. It also means the variation during PCB
manufacture results in greater impedance variation. Relative velocities of
differential and common components of the signal are sensitive to
separation between the signals (the amount of coupling between them), here
wider means lower difference in signal velocity and thus is probably
better. So overall wider is generally better. As you say nothing of the
details of the application, the question of whether these effects matter in
your application cannot be commented on. If it turns out that for you none
of these effects are significant, tighter coupling is easier to route on
the PCB.
The main reason many people give for using tight coupling is that the
fields from other nearby sources of interference affect both traces equally
if they're in practically the same point in space - but the presence of the
reference plane changes all that and make it a void argument. Tight
coupling is only necessary for that reason when there is no plane; for
example, in a twisted-pair cable.
With SI it is always dangerous giving general "good practice" remarks
because there are often many exceptions to the rule - but if you have to
start with something it should probably be: Tight coupling is best when you
don't have the space / layer count to do otherwise, but wider is better if
you have the space available. Others will probably add to the list of
differences, reasons why one is better than the other, and also point out
exceptions to the rule.
Regards
Jonathan
On 25 November 2014 at 21:38, Aaditya K <aaditya.kandibanda@xxxxxxxxx>
wrote:
> Hello Experts,
>
> I need opinions on instances where Loosely Coupled differential traces are
> better than Tightly coupled ones?
>
> Thanks
> Aaditya
>
>
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