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

L D Miller (ldmiller@nortelnetworks.com)
Thu, 29 Jul 1999 11:43:34 -0700

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.

(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.

Hope this helps,

Larry Miller
Nortel Networks

>
>
>
>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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