Thanks for the info Aubrey.
Interesting topic.!!
-----Original Message-----
From: Aubrey Sparkman [mailto:asparky@xxxxxxxxxxxxxxxxxxxxxxxx] ;
Sent: Monday, May 14, 2018 2:05 PM
To: Grasso, Charles <CHARLES.GRASSO@xxxxxxxx>
Cc: tgsmith81@xxxxxxxxx; Lee Ritchey <leeritchey@xxxxxxxxxxxxx>; Adam Dixon
<lanterna.viridis@xxxxxxxxx>; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Fwd: Differential and single-ended impedances
This message originated outside of DISH and was sent by:
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Charles,
Polar is a very good tool for figuring out pressed thickness, but there are
special steps to take if you have flex bondply instead of prepregs.
Sent from my iPhone
On May 14, 2018, at 9:10 AM, Grasso, Charles <CHARLES.GRASSO@xxxxxxxx> wrote:
Our company INSISTS that our pwb vendors use the Polar suite of tools
before we do business with them which is incidentally that same tool set we
use internally. A lot off error is squeezed out ..
Thanks
Charles Grasso
(w) 303-706-5467
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Tim Smith
Sent: Sunday, May 13, 2018 7:15 PM
To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
Cc: Adam Dixon <lanterna.viridis@xxxxxxxxx>; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Fwd: Differential and single-ended impedances
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 couple a
differential pair, each line will drive the impedance of the other
down and you will have to narrow each trace to get back to 50 ohms or
100 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
for some reason you cannot do that and they are separated as when
going through a BGA array, the impedance will increase dramatically.
When gain experience in this area, you will learn that differential
impedance is not the important driver. Two good 50 ohm lines is. We
learn that a "not closer than" rule avoids all of these problems. By
not closer we mean that the lines are far enough apart that they
don't drive the other's impedance down.
