[SI-LIST] AW: Re: AW: AW: Asymmetric differential stripline impedance
- From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
- To: SI-List <si-list@xxxxxxxxxxxxx>
- Date: Mon, 7 Jan 2013 16:14:11 +0000
Just to make myself clear:
When I said: "The Processing has so much influence, that I usually just push my
PCB maker as far as I can to a point where he simply can't mess up my design,
and then I give him the freedom to implement his processing tolerances."
I meant that I specify everything needed to meet the desired performance
(that's exact prepreg style, glass style, material, copper roughness,
copperweight, laminate and prepreg position in the stack, layer registration).
That's much more than just what's needed for impedance, but well enough that
the PCB maker can't vary things that would influence the performance. The only
thing he needs to do is to match the etching tolerances with his pressing
tolerances to meet the impedance.
Knowing that the etch might be adjusted is important as I have to use loose
enough coupling to allow for modifications (if diff pair spacing is already at
the producible limit, this wouldn't work out).
And especially for "beginners" it is the better choice to let the PCB maker
take care about the impedance, and learn from his feedback and from literature
to become an SI-PBC expert.
BR
Gert
----------------------------------------
Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339 Espelkamp;
Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB 8808;
Vertretungsberechtige Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning,
Dipl.-Ing. Torsten Ratzmann, Dr.-Ing. Alexander Rost
-----Ursprüngliche Nachricht-----
Von: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]
Gesendet: Montag, 7. Januar 2013 16:55
An: leeritchey@xxxxxxxxxxxxx; Havermann, Gert; SI-List
Betreff: RE: [SI-LIST] Re: AW: AW: Asymmetric differential stripline impedance
As I've posted in this forum before, I'm more in Gert's camp. The best you can
do, in my experience, is to get close to the target impedance and allow your
manufacturer to make small (<0.5mil is my rule of thumb) changes in trace
dimensions and/or dielectric thicknesses to meet the impedance spec. What I
typically find is that most vendors approximately match my impedance estimate
within those limits. Those that don't usually have a fundamental flaw in their
modeling assumptions (incorrect assumption about whether the top dielectric
thickness is from the top or bottom of the trace, different Er assumption,
etc.), not an inaccurate modeling tool.
As Gert said, to get more precise correlation you would need information from
cross-sectioning actual traces after they've been built. If this tool gets
within a couple of ohms in most cases, that would probably be sufficient for
many cases.
To me, this is a separate issue than whether I specify a particular glass style
or copper finish, and I'd like have them (the fab shops) own these in an
intelligent/informed fashion also.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Lee
Sent: Friday, January 04, 2013 10:07 AM
To: Gert.Havermann@xxxxxxxxxxx; SI-List
Subject: [SI-LIST] Re: AW: AW: Asymmetric differential stripline impedance
That's the way we have done this work for quite a while- let the fab shop
design the final stackup. Unfortunately, there is far more to designing a
stackup than just impedance, such as glass style, copper finish, etc. Most of
these, if not all are out of the scope of a fab shop's skill set. For this
reason, most of use have no choice but to take charge of the impedance
calculation as opposed to putting the burden on the fab shop. It was always
our job anyway! We just got away with forcing the fab shop to do it.
For these reasons, it is necessary to get impedance right before sending the
stackup off to a fab shop. That is why I argue for field solver based
calculators and good laminate data.
-----Original Message-----
From: Havermann, Gert
Sent: Friday, January 04, 2013 8:57 AM
To: Lee ; SI-List
Subject: [SI-LIST] AW: AW: Asymmetric differential stripline impedance
Lee,
I use it to find a starting point for 3D simulations. It can't be accurate as
one can not calculate with different prepreg styles or other real world
problems. But besides that its not a bad tool.
I think the biggest error in the end is the implementation of the design, and
there even 3D simulation doesn't help you if you don't know how PCBs are made
and which are the Materials used (not only electrical models, but also
processing related like flow behavior during pressing, copper fill of the
layer...). You know what I'm talking about.
If I have all the knowledge, then I take those parameters into my calculation
and do a 3D simulation of the right offset and the correct GND-GND distance and
correct material and surface roughness... And then I know that the impedance
will be at least close to where I wanted it to be.
For others without this detail of knowledge of PCB Fab and Materials, This tool
is a good starting point for their design. Then you hand this design to a pcb
vendor with knowledge in Impedance manufacturing and let him do the fine tuning
of the trace based on his experience to meet +/-10% impedance match. If The
vendor fails, he looses money, and you loose time, but with the right partner
it works great even at higher speeds.
The Processing has so much influence, that I usually just push my PCB maker as
far as I can to a point where he simply can't mess up my design, and then I
give him the freedom to implement his processing tolerances.
And quite frankly, if someone here asks for a tool that can do "offset
stripline", then I don't expect this person to know all the PCB processing
details yet, as I haven't seen any absolute symmetrical diff-trace in my life.
In reality they are all offset (sometimes a bit, sometimes a bit more).
You are absolutely right, that no one should ever trust a free tool up to a
point where mistakes can cost money. But free tools can be supportive.
BR
Gert
PS: will you have a DC Booth again this year? If so, I'll stop by for a talk.
----------------------------------------
Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339 Espelkamp;
Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB 8808;
Vertretungsberechtige Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning,
Dipl.-Ing. Torsten Ratzmann, Dr.-Ing. Alexander Rost
-----Ursprüngliche Nachricht-----
Von: Lee [mailto:leeritchey@xxxxxxxxxxxxx]
Gesendet: Donnerstag, 3. Januar 2013 18:38
An: Havermann, Gert; SI-List
Betreff: Re: [SI-LIST] AW: Asymmetric differential stripline impedance
Polar's CITS25 is an equation based tool that is accurate only part of the
time. It does not use a field solver. I don't trust its results.
As with most free tools, they are often worth the price!
-----Original Message-----
From: Havermann, Gert
Sent: Wednesday, January 02, 2013 11:54 PM
To: SI-List
Subject: [SI-LIST] AW: Asymmetric differential stripline impedance
Polar's CITS25 software is quite accurate and easy to use. It comes with
several different impedance cells. Even though Polar is no longer supporting
this tool, you will still be able to find the free evaluation version somewhere
in the web. The only evaluation restriction is the total number of
calculations, and it can be reset with uninstalling and re-installing.
BR
Gert
----------------------------------------
Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339 Espelkamp;
Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB 8808;
Vertretungsberechtige Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning,
Dipl.-Ing. Torsten Ratzmann, Dr.-Ing. Alexander Rost
-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Hithesh
Gesendet: Donnerstag, 3. Januar 2013 06:04
An: SI-List
Betreff: [SI-LIST] Asymmetric differential stripline impedance
Hi foks,
Is there any online tool to calculate the impedance of assymetric differential
stripline?
I searched, nothing available for asymmetric differential. It's either
asymmetric stripline or symmetric differential.
How to calculate differential impedance from single ended impedance?
This is with reference to USB signals.
Thanks
-Hithesh
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