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

L D Miller (ldmiller@nortelnetworks.com)
Thu, 29 Jul 1999 11:49:35 -0700

(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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