RE: [SI-LIST] : Re: (2) Positive ECL (PECL) voltage swings, but d

Knighten, Jim L (JK100005@exchange.SanDiegoCA.NCR.COM)
Thu, 29 Jul 1999 13:51:54 -0700

Larry,

Please explain your "these coupling caps be located in the fiber optics
transceiver package for reduced EMI" comment.

Jim Knighten
________________________________________________________
Dr. Jim Knighten e-mail: Jim.Knighten@SanDiegoCA.NCR.com
<mailto:Jim.Knighten@SanDiego.NCR.com> =20
Senior Consulting Engineer
NCR
17095 Via del Campo
San Diego, CA 92127 http://www.ncr.com <http://www.ncr.com> =20
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-----Original Message-----
From: ldmiller@nortelnetworks.com
[mailto:ldmiller@nortelnetworks.com]
Sent: Thursday, July 29, 1999 11:50 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Re: (2) Positive ECL (PECL)
voltage swings, but designed to be AC coupled

(Sorry, I hit the SEND button prematurely)

I believe I can answer some of that, at least for 1 Gb
Ethernet.

(A) The input circuits for fiber optics transceivers and
SERDES inputs need
to be common-mode DC biased to set the input stage operating
point. In
"classical PECL" this would normally be Vcc-2.0 VDC.
However, the various
transceiver manufacturers and SERDES manufacturers do not
agree on what
this voltage should be.

Therefore, if you use a "classical" 50-ohm
thevenin-equivalent DC circuit
(usually 82 ohms and 130 ohms) many combinations of fiber
optics
transceiver and SERDES are not happy, ranging from poor
noise margins to
total non-operation.

Since virtually all of these ICs have internal bias
generators we have
found that the best thing is to simply AC couple the ciruits
and let each
component go to its own DC "sweet spot".

(B) We have always insisted (and the manufacturers seem to
slowly be coming
around to this as a standard) that these coupling caps be
located in the
fiber optics transceiver package for reduced EMI. Also, the
SERDES
manufacturers all seem happy with using a pair of 50 ohm
resistors on their
inputs, with the center of the resistors tied to a 0.1 uF
capacitor to
produce a 50-ohm AC termination and let the IC set its own
DC operating point.

Hope this helps,

Larry Miller
Nortel Networks

At 11:28 AM 7/29/99 -0500, you wrote:
>
>
>
>All,
>
>Will some one shed some light on why differential lines (to
and from) a ECL
>device are AC coupled? I've been unsuccessful at finding
an answer from my
>internet searches.
>
>I am speaking specifically to the AC coupling that is found
in fibre-channel
>ser-des transmitters.
>
>(P)ECL C1
>|\-------------o-----||---
>| \ |
>| / | C2
>|/o------o-----------||---
> | |
> | |
> < R1 >R2
> > <
> | |
> | |
> _|_ _|_
> GND GND
>
>Where C1=3DC1=3D0.01uF
>
>Why is the capacitor needed? What is the theory behind
this implementation?
>
>Thanks,
>
>--------------------------------------------------------
> _ __ Salvador Aguinaga Jr.
>/_/ )/ _| Carrier Systems Group=20
>=B7 R&D
>__ |( (_ _ _ Mt. Prospect, Illinois
>\ \_)\__|(_)|)) (847) 222=B72833
>
>

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