[SI-LIST] Re: Propagation delay difference
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "jbtera77gmail.com" <jbtera77@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Fri, 9 Mar 2007 07:51:55 -0800
This may be true for pairs that are very tightly coupled to each other
because significant amounts of the fields couple between the two lines and
in a PCB, the dielectric between the two traces in primarily resin with an
er at or below 3 while between a trace and plane there is both glass and
resin with an er around 4.
However, there is no benefit to tightly coupling the two members of a pair
when they are routed in a PCB. On the contrary, there are several
disadvantages. When the two lines are separated enough to call them
loosely coupled, this problem goes away.
There is another problem with diff pairs that has to do with velocity.
This is the subject of much debate right now and that is timing skew
between two members of the same pair. This is traceable to the irregular
distribution of fibers in some glass such as 1080 and 106. Papers have
been published about this problem and some have proposed routing signals at
an angle to the weave to solve the problem. There is at least one patent
being appplied for on this idea.
This solves the problem, but at a huge penalty in PCB cost.
A much simpler solution will be shown in an article I wrote that will be in
the April issue of Circuitree.
Lee Ritchey
Speeding Edge
> [Original Message]
> From: <jbtera77@xxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 3/9/2007 3:36:55 AM
> Subject: [SI-LIST] Propagation delay difference
>
> Hi, all
> As all know, in homogeneous strip line, the propagation speed is the same
> for both differential and common mode.
>
> However, in inhomogeneous microstrip line, the propagation speed of
> differential mode is faster than that of common mode.
> In specific, the value of Cm/C is higher than Lm/L in inhomogeneous
> microstrip line while the values are same in homogeneous strip line?
>
> Could somebody let me know why it physically is?
>
>
> Thanks in advance,
> Jongbae Park.
>
>
>
> --
> --------------------------------------------------------Ph.D Candidate
> StudentTerahertz Interconnection & Package Laboratory,Department of
> Electrical Engineering, Korea Advanced Institute of Science and Technology
> (KAIST),373-1, Kusong-dong, Yusong-ku, Taejon 305-701,
> KoreaFax)+82-42-869-8058Tel)+82-42-869-5458Mobile)+82-11-9787-7966E-mail)
> pjb77@xxxxxxxxxxxxxxxxxxxxxx URL)
>
http://tera.kaist.ac.kr/----------------------------------------------------
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