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

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From: Moran, Brian P ([email protected])
Date: Tue Apr 03 2001 - 14:14:37 PDT


Larry,

I do not disagree that standard FR-4 can work well in many applications
above 1 GHz. However, I generally preferred to play it a bit safe, when
cost is secondary to reliability or time to market. I seem to remember
finished board cost premiums for GETEK closer to 25%. Well worth the piece
of mind. At 80% your right to be selective. And its never a good idea to
limit your vendor or material options unless its truly neccessary.

It also depends on application. Single bit clock encoded interfaces can
withstand a good deal of rolloff, but the x4, x8, and x12 SERDES and PAROLI
interfaces which have clock encoded on only one bit require some care in
maintaining edge shape so as to minimuize skew across the full interface. I
have debated this tradeoff with myself many times, but when I'm feeling
conservative I try to maintain signal bandwidth out to the 5th harmonic if
possible, which gets tough with standard FR-4 running at 2.5Ghz, unless
traces are very very short. I'm sure I'll hear some disagreement on this,
but thats my take.

 

Brian P. Moran
Signal Integrity Engineering
Desktop Products Group, Intel Corp.
Mail-Stop: FM6-45
1900 Praire City Rd, Folsom CA 95630
Email: [email protected]
Phone:(916) 356-1912

-----Original Message-----
From: Larry Miller [mailto:[email protected]]
Sent: Tuesday, April 03, 2001 12:04 PM
To: 'Michael Nudelman'; [email protected]
Subject: RE: [SI-LIST] : RE: 2.5 GHz in FR4

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:[email protected]]
Sent: Tuesday, April 03, 2001 10:20 AM
To: [email protected]
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:[email protected]]
Sent: Tuesday, April 03, 2001 12:57
To: 'Gaboian, Jerry'; 'Zabinski, Patrick J.'; [email protected]
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:[email protected]]
Sent: Tuesday, April 03, 2001 5:00 AM
To: 'Zabinski, Patrick J.'; [email protected]
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:[email protected]]
Sent: Tuesday, April 03, 2001 6:24 AM
To: [email protected]
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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