[SI-LIST] Re: delay tuning method of differential pair
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "shlepnev@xxxxxxxxxxxxx" <shlepnev@xxxxxxxxxxxxx>, "pcbtime@xxxxxxxx" <pcbtime@xxxxxxxx>, 'si-list' <si-list@xxxxxxxxxxxxx>
- Date: Fri, 22 Nov 2013 16:23:01 +0000
In my experience, the recommendations for differential pair phase compensation
stems as much from implementation ease as electromagnetic optimization.
Perhaps for a small, very high frequency design (or a small portion of a larger
design), it's best to carefully optimize every turn of your differential pair.
For a complex design with thousands of differential pairs, you're forced to use
rules which are easy to implement and check. This was especially true before
CAD tools automated phase-matched routing in their software. Here are some of
the major considerations I know of:
1) Several small, short serpentines are usually easier to implement than a
single big loop. Often you don't have the room for a big loop where you need
the compensation.
a. If you were doing a big loop, you'd also have to ensure lots of
separation between adjacent "legs" - this would be very large for microstrip.
2) I like to have half of the compensation done before turn(s), and half
afterwards. I think this makes the most sense electromagnetically, and also
makes it the easiest to check (I don't have to figure out which direction a
signal is going in order to figure out if it's done correctly).
a. One exception here is if there are compensations needed for skew
within a package or connector.
3) While the amount of skew allowed probably depends on the datarate for
electromagnetics, it might be simpler to use the same rule for all (or many) of
your differential busses. It makes checking easier and avoids another chance
to mistype a constraint.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Yuriy Shlepnev
Sent: Tuesday, November 19, 2013 10:04 AM
To: pcbtime@xxxxxxxx; 'si-list'
Subject: [SI-LIST] Re: delay tuning method of differential pair
When estimating different skew compensation structures, in addition to
comparison of SDD11 and SDD21, you have to compare the mode transformation
parameters SDC11, SDC22 (near-end mode transformation) and SDC12, SDC21
(far-end mode transformation). In general, skew compensation for coupled
differential pairs must be formulated not in terms of the length matching, but
rather as minimization of the differential reflection and near and far end mode
transformation parameters. See example at this presentation
www.simberian.com/AppNotes/Shlepnev_DesignInsights_FRSI_Oct3_2013.pdf<http://www.simberian.com/AppNotes/Shlepnev_DesignInsights_FRSI_Oct3_2013.pdf>
(starting from slide 29). As you can see, addition of serpentines for some
structures may not improve the "skew" and make things worse. Do comprehensive
analysis with the tools that capture the relevant effects.
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
3030 S Torrey Pines Dr. Las Vegas, NV 89146, USA Office +1-702-876-2882; Fax
+1-702-482-7903 Cell +1-206-409-2368; Virtual +1-408-627-7706
Skype: shlepnev
www.simberian.com<http://www.simberian.com>
Simbeor - Accurate, Fast, Easy and Affordable Electromagnetic Signal Integrity
Software
2010 and 2011 DesignVision Award Winner
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
[mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of pcbtime
Sent: Tuesday, November 19, 2013 2:33 AM
To: si-list
Subject: [SI-LIST] delay tuning method of differential pair
Hi Expert
I was confuse about the delay tuning method. We have two choice: Single
loop or small serpentines Single loop can match the length quickly then
small serpentines. That's mean:
Single loop has big different when tuning but short time.
small serpentines has small different when tuning but takes long time.
We try to simulation the result is below 12.5GHZ. We can't find any
different in SDD21 and SDD11 That's mean Single loop and small serpentines
is almost same when signal is below 8GBps Why most the design guide force to
use small serpentines
And I am so sorry for send two mail with unreadable code
Best Regards
pcbtime------------------------------------------------------------------
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