[SI-LIST] Re: Diff line
- From: Ben Rothchild <benrothchild@xxxxxxxxx>
- To: scott@xxxxxxxxxxxxxxxx
- Date: Fri, 15 Feb 2002 03:40:18 -0800 (PST)
Hi Scott,
I have been on the silist for a little
while now. This discussion is interesting.
I have not worked on gigabit serial links
however I will be working on them in the
next project(atleast hope so).
Quite frnakly I didn't understand many
of the issues you mentioned in your
mail. Could you please point me to some
reference where these issues are discussed.
Ofcourse I have embedded my doubts in your
mail below. I would be extremely grateful
if you could kindly clarify them.
Regards,
Ben
--- Scott McMorrow <scott@xxxxxxxxxxxxxxxx> wrote:
>
> All,
>
> Both loosely coupled and tightly coupled
> differential pair designs work
> as all authors have previously stated. The
> following is a list of some
> of the general differences between the two types of
> edge-coupled
> differential pairs:
>
> Loosely coupled
> better impedance control
doubt:
How is impedance control dependent on whether
they are loosely/tightly coupled.
> less sensitivity to manufacturing variations
> lower track density
> lower even mode return loss
doubt:
What is even mode return loss and how is coupling
affecting this.
> lower insertion loss
doubt:
How is insertion loss affected by coupling.
> less impedance variation at 2mm connector
> transitions
doubt:
Are you experts there driving gigabit links
through 2mm open pin field connectors!
Its very hard to believe. Or is it controlled
impedance connectors. But then are there any
controlled impedance 2mm connectors. Any
inputs on what connectors you are using and
the highest data rate.
> less impedance variation at BGA escapes
> transitions
> less impedance variation at via transitions
doubt:
How is impedance variation at connector, BGA
escapes and via transitions affected by
coupling.
>
> Tightly coupled
> worse impedance control
> greater sensitivity to manufacturing variations
> higher track density
> higher even mode return loss
> higher insertion loss
> greater impedance variation at 2mm connector
> transitions
> greater impedance variation at BGA escape
> transitions
> greater impedance variation at via transitions
>
> As with all things in interconnect engineering,
> there are tradeoffs. At
> 3.125 Gbps and above, it's the impedance mismatches
> that get you into
> trouble, especially near connectors, vias and device
> escapes. It is
> extremely easy to create unwanted resonances which
> will cause unexpected
> losses at the strangest frequences, some easily
> within the frequency
> spectrum of data streams.
doubt:
What resonance are you refering to, could you
please elaborate.
>
> For high speed circuits where trace density is not
> an issue, I always
> choose to use loosely coupled differential pair
> conductors, where
> impedance control is much more precise, and
> resonances caused by
> impedance mismatch can more easily be reduced.
>
>
> regards,
>
> scott
>
>
> --
> Scott McMorrow
> Principal Engineer
> SiQual Interconnect Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231
> http://www.siqual.com
>
>
>
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