Perhaps someone could do a test board with two sets of
signals, one "twisted" as described and the other done
in the more traditional way. Make measurements and
post your results :-)
--- Matthew Lamkin <[log in to unmask]>
wrote:
> I'd suggest that UNLESS the engineer can come up
> with hard evidence that
> this is good practise then you should stick to your
> guns and refuse to do
> it. As evidence to the contrary is available to
> prevent you wasting your
> time & the cost involved in up-issuing & retesting
> the boards.
>
>
>
> -----Original Message-----
> From: Dennis Ward
> [mailto:[log in to unmask]]
> Sent: Wednesday, November 19, 2003 1:36 PM
> To: [log in to unmask]
> Subject: Re: [DC] Twisting Differential pairs
>
>
> Thank you for all your replies. I'm glad I'm not the
> only one who thought
> this was
> not good design practice.
>
> Thanks for the links, I've been to these sites,
> checked the archives on the
> IPC site,
> did a google search on "differential pair routing",
> "differential pair",
> etc... and quite
> honestly cannot find an article on this twist. Not
> to single you out Denis,
> but an reference
> would be great to look at. The engineer said the
> same thing, he saw an
> article recommending
> this but can't find the article right now.
>
> If the engineer comes up with any hard info on this
> I will send it out under
> this subject heading
> so it can be archived.
>
> Dennis
>
> -----Original Message-----
> From: DesignerCouncil
> [mailto:[log in to unmask]]On Behalf Of Denis
> Lefebvre
> Sent: Tuesday, November 18, 2003 5:53 PM
> To: [log in to unmask]
> Subject: Re: [DC] Twisting Differential pairs
>
>
> Don't freak out, Patrick et al.
> I have seen some models of this and articles as
> well. (sorry, can't
> quote right now - I just remember seeing them - it
> may have been the
> CID+ material) My take is that its one of those
> things that looks good
> on paper - but show me some emperical data. Before
> commiting to it, I'd
> ask the engineer to do a "test coupon" to verify if
> it'll work, and if
> so, how well.
> Just my $.02
>
> Denis Lefebvre, CID+
> Senoir PCB Designer
> Finisar Corp.
>
> -----Original Message-----
> From: Patrick Jabbaz [mailto:[log in to unmask]]
> Sent: Tuesday, November 18, 2003 2:47 PM
> To: [log in to unmask]
> Subject: Re: [DC] Twisting Differential pairs
>
>
> Dennis
> You must be kidding!
> It sounds that the Engineer is thinking in terms of
> cabling no external
> reference. Differential signaling on a twisted
> cable, you get excellent
> coupling, however on a PCB it is pretty lousy, even
> if you spaced them
> very close together. You must also consider that
> Broadside differential
> routing, example L2 AND L3. The layers must be cores
> in order to
> maintain good registration layer to layer. The best
> choice that I
> recommend is to use edge couple side by side with a
> good reference and
> match the skew within the pair since so will not get
> very good coupling
> on the diff signals +
> -
>
> There are several good articles on differential
> routing go to
>
> http://www.sigcon.com/pubsChron.htm
>
>
> Inkra Networks
> Patrick Jabbaz CID
> Sr Board Layout Eng.
> 40971 Encyclopedia Circle
> Fremont, CA 94538
> Work (510) 249-4835
> Mobile (408) 621-6533
> [log in to unmask]
>
>
> -----Original Message-----
> From: Dennis Ward
> [mailto:[log in to unmask]]
> Sent: Tuesday, November 18, 2003 2:36 PM
> To: [log in to unmask]
> Subject: [DC] Twisting Differential pairs
>
> Good evening,
>
> I wonder if anyone has any experience, insight,
> opinion on this subject.
> The engineer would like his longer (> 6")
> differential pairs "twisted".
> Basically run the pair in a dual strip line (pairs
> running on top of
> each other) when the pair reaches 4 inches swap
> layers on the pair.
>
> So for example - say we have a 6 layer card. L1 is
> top, L2 is GND L3
> signal, L4 signal, L5 VCC and L6 bottom.
> Differential pair (lets call it
> Diff
> pair)
> running
> on Signal 3 and Signal 4. Diff Pair+ on Signal 3,
> Diff Pair- on Signal
> 4. When the pair reaches 4 inches swap layers. Now
> Diff pair- on Signal
> 3 and Diff
> pair+
> on Signal
> 4. Repeat layer swap every 4 inches until end of
> net.
>
> The only caveat he has on this is that this is not
> to be done on Gigabit
> lines. Other than that he would like to see this
> model on his
> differential routing.
>
> I have not heard of this nor can I find any info on
> this. I'm curious if
> anyone is doing this. Is this accepted practice? Is
> it beneficial?
>
> Thanks for sharing your experience on this matter
>
> Dennis
>
>
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