Sorry, although your wide-scope monologue is interesting and got me thinking, I
donât see how it was âclarifyingâ as it relates to material properties.
I presented a simple and practical process to evaluate materials suited for
32Gbpsec level test fixtures and fabrications, that is all.
Furthermore, there is no such thing as âskewâ. There is only velocity of
propagation versus frequency and a mode for that propagation. None of the
microwave, transmission line, and RF books I have include âskewâ in the
appendix. Skew was a 1nsec rise time logic system thing from the 1980âs.
Secondly, group delay directly impacts the jitter of an NRZ signal, based on
the power spectral density, in a predictable way. I donât see a way of
doing that using phase delay, feel free to educate us on the practicalities of
that methodology.
I agree that group delay does aggravate noise in a stochastic system, but it is
not invalid method, I recall it is used extensively for causality analysis in
most EDA packages. We use it all the time to predict the bandwidth lossed when
de-embedding. I can predict the effective bandwidth of a post processed
de-embedded system quite accurately using that data. Making good group delay
measurements isnât difficult if you set up the VNA correctly btw.
- Al
Products for the Signal Integrity Practitioner
Alfred P. Neves
Chief Technologist
Office: 503-679-2429
www.wildrivertech.com <http://www.wildrivertech.com/>
2015 Best In Design&Test Finalist
On Dec 30, 2015, at 1:01 PM, Scott McMorrow <scott@xxxxxxxxxxxxx> wrote:
To clarify a bit
1) Skew ... in laminates, in trace routing, in packages, in drivers, and in
asymmetric 3D transitions is the primary physical problem that is occurring.
2) Skew is primarily measured in the frequency domain as phase delay. Group
delay is a derivative function and is subject to measurement noise,
resonances, and numerical error.
3) Skew causes conversion from the differential to the common mode of
propagation (Scd), and from the common mode to the differential mode (Sdc).
4) Skew is either uncontrolled (laminate weave) or controlled (trace breakout
routing, asymmetric via transitions).
5) Controlled skew may be recovered if, and only if, there are no TEM
discontinuities between the point of skew creation and the point of skew
compensation.
6) Uncontrolled skew cannot be compensated for by definition. It exists as a
statistical anomaly for common materials.
7) When controlled or uncontrolled skew reaches a TEM discontinuity, both the
differential mode and the common mode scatter.
8) The scattering of common mode energy results in significant propagation of
crosstalk energy between vias, which are not well localized for common mode.
9) Common mode scattering is almost always asymmetric, resulting in
additional common mode to differential mode noise conversion in via fields.
(i.e. Crosstalk)
10) Common mode scattering into planes, being poorly localized by ground
vias, tends to result in noise "pick up" across the entire PCB, and is highly
sensitive to all cavity resonances.
11) There are very simple test methods that can be used to determine the
relative skew sensitivity of different laminates, which can be incorporated
into Generalized Modal S-Parameter material characterization methods.
12) Test boards have the advantage of being able to use special materials,
better weaves, isotropic materials, or routing tricks for skew mitigation.
With the exception of better weaves, most of these techniques are not viable
for design and manufacture of real systems, due to economic concerns.
Ultimately, we are reaching a final bound on the applicability of PCB
materials for use in the ever-increasing interconnect bandwidth, due either
to the inherent skew of real materials, or the inherent loss limits of these
materials and traces.
We are at an inflection point in high performance interconnects. Either we
turn to optical interconnect, or to something else that is copper.
Personally, I have the opportunity to work in both the optical and the copper
electrical playground. This next generation of 56G NRZ and beyond will be
designed with Twin-Ax cable, both on-board, on-backplane, and within rack.
Scott McMorrow
R&D Consultant
Teraspeed Consulting - A Division of Samtec
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com <http://www.teraspeed.com/>
On Wed, Dec 30, 2015 at 3:27 PM, Alfred P. Neves <al@xxxxxxxxxxxxxxxxx
<mailto:al@xxxxxxxxxxxxxxxxx>> wrote:
âSkewâ IMHO is not a good metric for 50GHz level signal integrity (32G
NRZ, 56G PAM-4). I realize this may be slightly veering from the initial
thread, but it relates to what is going on with IEEE p370 activity and
relates keeping the eye open.
For our Custom Channel Modeling platforms we use the following metrics for
fabrication and material performance:
1. Variation of impedance trace to trace, where the impedance is an
average, this relates to the typical 5% impedance tolerance
2. Variability of trace impedance using a fast TDR, or a measure of
homogeneity of the material for a single trace. Traces need to be rotated
from in line with weave to orthogonal. Poor homogeneity, such as 2-3ohms,
for 15-45 degree rotations is really not a good thing. We will probably
start identifying specific laminate systems and fabricators with a 3sigma
number and histogram the variation as well.
3. Group delay variation from DC to Fmax s21, s43 and SDD21. This is a
much better metric than skew, especially for microstrip transmission lines or
test fixtures mixing single and differential.
4. Differential to common mode, SDC21 transformation for carefully balanced
systems (no bends or twists). The device/package can potentially generate
skew based on poor return loss or mode issues as well.
5. Group delay noise, SDD21, or s21 and s34 or forward transmissions.
We find that simulation to measurement correspondence, material
identification, and the commonly used de-embedding schemes (AFR for example)
work out much better using this comprehensive look at materials. Yuriy
Shlepnev suggested some time back looking at both real/imaginary parts of the
s-parameter but I havenât got my head around that yet. We are using
cross section for VIAâs , trace geometries, but not for X-Y yet, thanks for
the idea Lee and Bill. The X-Y cross section will juxtapose nicely against
the electrical verification.
Any positive comments or other ideas?
- Al Neves
Products for the Signal Integrity Practitioner
Alfred P. Neves
Chief Technologist
Office: 503-679-2429
www.wildrivertech.com <http://www.wildrivertech.com/>
<http://www.wildrivertech.com/ <http://www.wildrivertech.com/>>
2015 Best In Design&Test Finalist
On Dec 30, 2015, at 9:09 AM, Lee Ritchey <leeritchey@xxxxxxxxxxxxx
<mailto:leeritchey@xxxxxxxxxxxxx>> wrote:
I polish two of the edges, one X and one Y and use a microscope to see what
the glass styles are.
Since I want to know before I accept delivery, I require the fabricator to
include a copy of the traveler with the shipment. This shows what glass
styles were used.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <mailto:si-list-bounce@xxxxxxxxxxxxx>
[mailto:si-list-bounce@xxxxxxxxxxxxx <mailto:si-list-bounce@xxxxxxxxxxxxx>]
On Behalf Of Bill Hargin (Nan Ya, USA)
Sent: Monday, December 28, 2015 12:26 PM
To: bbakshan@xxxxxxxxx <mailto:bbakshan@xxxxxxxxx>; si-list@xxxxxxxxxxxxx
<mailto:si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Glass Weave effects and Cross sectioning
Hi Boris:
If the boards have already been built, and you know the relative dimensions
you're looking for with respect to the yarn width and height, you could do
that. If it were me, I'd first work backward from the fabricator to the
laminate manufacturer first - to see what kind of information I could get
on whether spread glass was used. In most cases, you should at least be
able to get a yes/no answer on that. It's less likely, but some
fabricators/laminate manufacturers will even be able to provide specific
cross-section dimensions for various weave constructions.
Armed with whatever information you can get for free, you can certainly do
some cross sectioning and SEM measurements, if you have the means, and you
have an idea regarding the aspect ratios you're looking for.
I have this kind of data for our fabric, which is all spread, as a general
practice. I believe that Isola provides this info for their glass
constructions, as well. Let me know if that data would help, and I'll
provide it offline.
Out of curiosity, have you had (glass weave) skew problems, or is it just a
concern at this point.
Bill Hargin
Director of North American Sales and Marketing Nan Ya Copper-Clad Laminates
billh@xxxxxxxxxxxxx <mailto:billh@xxxxxxxxxxxxx> ⪠425-301-4425 âª
Skype: bill.hargin
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <mailto:si-list-bounce@xxxxxxxxxxxxx>
[mailto:si-list-bounce@xxxxxxxxxxxxx <mailto:si-list-bounce@xxxxxxxxxxxxx>]
On Behalf Of Boris Bakshan
Sent: Sunday, December 27, 2015 10:17 PM
To: si-list@xxxxxxxxxxxxx <mailto:si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Glass Weave effects and Cross sectioning
Hi experts,
I've been reading much on glass weave effects and its contribution on skew.
If I encounter a skew problem, can I say anything on weave type ,whether it
is mechanically spread or not by doing a cross section of my traces?
If not, how can I guarantee that the right type of weave was indeed
selected when building my board?
Thank you all!
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