Hello - How do you know this free tool gives accurate results?
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Ching-Chao Huang
Sent: Sunday, May 13, 2018 9:59 PM
To: cristian.gozzi@xxxxxxxxx; 'Tim Smith' <tgsmith81@xxxxxxxxx>
Cc: 'Adam Dixon' <lanterna.viridis@xxxxxxxxx>; 'Lee Ritchey'
<leeritchey@xxxxxxxxxxxxx>; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Fwd: Differential and single-ended impedances
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The following 2D solver is free and it gives very accurate impedance:
http://ataitec.com/free_2d_solver/
Regards,
Ching-Chao Huang
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Cristian Gozzi
Sent: Sunday, May 13, 2018 8:54 PM
To: Tim Smith
Cc: Adam Dixon; Lee Ritchey; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Fwd: Differential and single-ended impedances
you can also create your own custom material if I remember well or select one
in the library that has properties closer to the one you are using I hope this
help Cris
On Sun, May 13, 2018 at 8:34 PM Tim Smith <tgsmith81@xxxxxxxxx> wrote:
Thanks Cris, I'll give it a shot, but it seems to only allow Rogershttps://www.rogerscorp.com/documents/8939/acs/2017-Microwave-Impedance-Calcu
material, funny about that...
On Mon, May 14, 2018 at 1:08 PM, Cristian Gozzi
<cristian.gozzi@xxxxxxxxx>
wrote:
Hi Tim
Have you ever tried the free tool provided by Rogers for calculating
the trace impedance?
the two
To me looked nice and much closer to 2D field solver impedance
calculator
Regards
Cris
On Sun, May 13, 2018 at 6:16 PM Tim Smith <tgsmith81@xxxxxxxxx> wrote:
Thanks Lee,
I guess that really validates a comment you one made that the value
provided by a free tool is equal to its cost...
I've seen may PCB designers rely heavily on these free tools (as
decent tools cost money), but their differential pair calculators
don't change the value produced for Zo as the spacing changes. This,
it would seem, is a severe error and completely invalidates the
result.
Why is it that companies expect these designers to produce quality
PCBs without investing the money to actually do so?
On Mon, May 14, 2018 at 11:01 AM, <leeritchey@xxxxxxxxxxxxx> wrote:
As mentioned in that cited paper, if you choose to tightly coupledown and
a differential pair, each line will drive the impedance of the
other
you will have to narrow each trace to get back to 50 ohms or 100for
ohms differential if you prefer to think that way. In the bargain
you will increase skin effect loss. However, that is not the big down
side.
Wherever the pair travels it will have to remain tightly coupled.
If
some reason you cannot do that and they are separated as whenthrough
going
a BGA array, the impedance will increase dramatically.learn
When gain experience in this area, you will learn that
differential impedance is not the important driver. Two good 50
ohm lines is. We
that a "not closer than" rule avoids all of these problems. Bycloser
not
we mean that the lines are far enough apart that they don't drivedifferential
the other's impedance down.
In closing, there is not real value in "tight coupling" in a
pair.rather
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx>
On Behalf Of Tim Smith
Sent: Sunday, May 13, 2018 5:46 PM
To: Adam Dixon <lanterna.viridis@xxxxxxxxx>; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Fwd: Differential and single-ended
impedances
Yes, theoretical only. I would not expect any fab shop to accept
that geometry.
I've been reading through one of Lee's papers, " A TREATMENT OF
DIFFERENTIAL SIGNALING AND ITS DESIGN REQUIREMENTS" and in it, the
following statement is made:
"The reason that differential impedance is almost always specified
than single-ended impedance is that it is thought that
differential impedance is different than the sum of the two
impedances when the individual impedance is measured from
ââ,¬Å"groundââ,¬ to either of
(iftraces.""differential"
Is it true that as we bring the two traces together both the
impedance and the single-ended impedance reduce so that the"differential"
is always 2x that of the single ended?such
If so, this is where simple tools such as Saturn PCB fall over.
Tools
as these seem to be heavily relied on due them being free.lanterna.viridis@xxxxxxxxx>
Should we really be specifying "odd-mode" instead of "single-ended"?
On Mon, May 14, 2018 at 10:23 AM, Adam Dixon <
wrote:
This is a theoretical question, I assume, as from a fabrication
perspective, 0.05mm spacing will be a major major yield
challenge
potentialany fabricator will accept producing the design reliably). You
have to know your receiver VIH/VIL/noise specs to understand the
breaksperformance impact or at what point you cross a threshold that
thethe channel.wrote:
Ultimately it's a pair of single-ended nets, each of which has
impedance matching and VIH/VIL requirements, right?
Regards,
Adam in Atlanta
adam.dixon@xxxxxxxx
On Sun, May 13, 2018 at 7:48 PM, Tim Smith <tgsmith81@xxxxxxxxx>
in
Thanks for the responses.
So, say I route the two traces with very tight coupling,
something
the order of only 0.05 mm between them. This will greatly
reduce
vswrote:"differential impedance".
Logic tells me that as long as each trace is still a 50R line,
the system will work, but what effect does reducing this
differential impedance actually have on the system performance?
On Sat, May 12, 2018 at 2:25 AM, Joel Brown <joel@xxxxxxxxxx>
Tim,due
As you have alluded, two traces routed close together will
have a lower differential impedance than two 50 ohm traces
routed far apart. This is
to the coupling between the traces. Routing a differentialclose
pair with
spacing has the benefit of higher noise immunity and lowerdue to
radiation
cancellation of the fields. A differential impedance of 97
ohms
tolerance100ohms
is not going to be significant. Most boards shops have a
(I211).routedwrote:of+/-
10% on controlled impedance traces. You could run simulationsthe
and vary
impedance from 90 to 110 ohms to see what effect it has on
the signal integrity.
Regards,
Joel
On Thu, May 10, 2018 at 9:29 PM, Tim Smith
<tgsmith81@xxxxxxxxx>
research of
Hi experts,
I'm aware that this subject has been well discussed, but in
my
inboth the SI list and many Google searches, I seem to have
found myself
a
bit of a muddle.
Let us consider a PCIe "differential" pair that needs to be
on a PCB from a processor (i.MX6) to a PCIe to GbE bridge
ondifferentialA "differential" impedance of 100R is called out as the I211
contains on-die terminations.
I know from reading some literature that some insist that
the
transmissionimpedance is not important and to just route them as two 50R
lines. This makes sense given the theory of the differential
signalling and the 100R termination.
Now comes the practical application of placing these two
traces
impedanceapart.the PCB.
There are two methods, route them close together, or route
them far
This is where I start to not be able to get a solid answer.
The math states that for these signals, the differential
modeis twice that of the odd mode impedance. We know that the
odd
tooimpedance is dependent on the traces separation. Therefore,
so
it'sdifferentialis the
single-endedimpedance.
the first-order approximation equations that many designers
use (those without the capabilities of 3D field solvers)
state that for two 50R single-ended transmission lines
operating as differential signals, you need infinite
separation in order to achieve 100R differential impedance.
In my designs, I've always focused more on targeting the 50R
butrather than the 100R differential.
I've heard many times that the differential impedance does
not matter,
my questions are these:
- Why is there so much emphasis placed on this concept of
differential impedance?
- Does if affect the performance of the signals in any way
if
------------------------------------------------------------------differential,not matched to the termination resistor(s)?
- If I target 50R single-ended and route them so I get 97.3R
do I need to adjust the termination resistor(s) to match?
Looking forward to your thoughts.
Dr. Tim
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