Re: [SI-LIST] : RE: 2.5 GHz in FR4

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Wed Apr 04 2001 - 08:16:15 PDT


I design backplanes as described by Larry. When we need to minimize dielectric
losses, we use Nelco 4000-13SI. It truly has a lower loss tangent that FR-4 or
Getek. Can't really understand why anyone uses Getek. Not much upside, lots of
down side.

Lee

Larry Miller wrote:

> Not at all masochistic.
>
> This was an extremely easy board to route. AMP, Erni and Teledyne (in no
> particular order) have expended great amounts of research for high speed
> back/mid planes using differential pairs for the telecomm and networking
> industries and have reduced it down to a turn-the-crank operation. (The app
> notes are readily available on their websites.)
>
> Plus, the quotes we have regularly gotten from backplane/midplane vendors
> show the cost for Getek to be about 1.8X that of FR-4, and the more exotic
> ones like the Rodgers materials go up exponentially.
>
> Thirdly, the other materials (Getek and up) have comparative availability
> problems when you are using slabs that are 17" or 21" wide by 24" or more
> high.
>
> Lastly, for serial backplanes, the SERDES manufacturers have fitted their
> chips up with pre-emphasis logic to hugely reduce the data-dependent jitter
> (DDJ) caused by the roll-off vs. frequency of FR-4.
>
> If anyone from Tyco or other such fabricators is listening, please chime in.
>
> Larry Miller
>
> -----Original Message-----
> From: Michael Nudelman [mailto:mnudelman@tellium.com]
> Sent: Tuesday, April 03, 2001 10:20 AM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : RE: 2.5 GHz in FR4
>
> This is quite masochistic. Do you have to do it to yourselves and use FR4,
> unless you are running couple of inches of hi-speed wires?
>
> -----Original Message-----
> From: Larry Miller [mailto:ldmiller@rhapsodynetworks.com]
> Sent: Tuesday, April 03, 2001 12:57
> To: 'Gaboian, Jerry'; 'Zabinski, Patrick J.'; si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : RE: 2.5 GHz in FR4
>
> I, too, must take issue with practically every statement made by Mr.
> Zabinski.
>
> We are using stripline. We do not have vias except the connector pins (AMP
> HS-3).
>
> We are using 10-mil stripline traces on the backplane, 5-mil stripline on
> the plug-in boards (have to, the boards get too thick otherwise). Yes, the
> 5-mil sections are lossy because of skin effect; luckily, these are short.
>
> We have backplane traces that are over 12" long (can't help it).
>
> In spite of all this, our system happily percolates along at 3.125 GHz (2.5
> GHz with 8B/10B coding).
>
> Larry Miller
>
> -----Original Message-----
> From: Gaboian, Jerry [mailto:j-gaboian@ti.com]
> Sent: Tuesday, April 03, 2001 5:00 AM
> To: 'Zabinski, Patrick J.'; si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : RE: 2.5 GHz in FR4
>
> Check out this site from AMP.....
>
> It will surprise you on how long of a trace one can use.....
> They also compare other materials, widths act.....
>
> http://www.amp.com/simulation/files/papers/dc99cmdh.pdf
>
> Regards,
> Jerry
>
> -----Original Message-----
> From: Zabinski, Patrick J. [mailto:zabinski.patrick@mayo.edu]
> Sent: Tuesday, April 03, 2001 6:24 AM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : RE: 2.5 GHz in FR4
>
> Sreejith,
>
> It is possible to use common FR4 in 2.5 GHz systems, but you
> need to follow a few design constraints:
>
> * Do not use stripline. The discontinuities presented by
> the vias are most often killers. Avoid all vias in the high
> speed nets and use microstrip (i.e., top-surface only).
>
> * Use wide traces. FR4 is lossy at 2.5 GHz, so try to compensate
> for the high dielectric loss by decreasing the metal loss. Use
> wider traces which will reduce the resistive losses in the metal.
>
> * Use short traces. If you need to route 2.5 GHz signals
> on nets that are 10 inches (25 cm) long, I doubt if your
> system will function. Keep the traces *short* (say, less
> than 1 inch [2.5 cm]).
>
> Hope this helps,
> Pat
>
> > HI All,
> >
> > i am doing a 2.5Ghz PCB design. i was wondering whether FR4 material
> > is sufficient for such high speed signals? or i should look at some
> > other material for these desings? and if FR4 is sufficient is
> > there any
> > special care i have to take (other than impedance matching
> > and differential
> > routings)
> >
> > thanks in advance,
> > Sreejith
> > Paxonet Comm. (I) Ltd.
> > Blore, India
>
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