RE: XAUI IO specs
The intention of Hari was 800 mV differential p-p when you use the scope A-B. As you said this
voltage is fictious and the receiver only sees 1/2 of swing. For the case of Hari XAUI the
pick-pick swing across the base of transistor would 400 mV ((VCM+200mV) - (VCM-200mV)).
> From: Stuart Brorson <sbrorson@xxxxxxxxxxx>
> To: kdemsky@xxxxxxxxxx, stds-802-3-hssg@xxxxxxxx, t11_2@xxxxxxx
> Subject: RE: XAUI IO specs
> Date: Fri, 28 Apr 2000 09:56:36 -0400
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> > By your own statement below, a static Logic 1 is when Vo+ is 400 mV above
> > Vcm, and Vo- is 400 mV below Vcm, then the differential voltage (Vo+) -
> > (Vo-) is 800 mV (positive peak).
> > By your own statement below, a static Logic 0 is when Vo+ is 400 mV below
> > Vcm, and Vo- is 400 mV above Vcm, then the differential voltage is (Vo+) -
> > (Vo-) is -800 mV (negative peak).
> > Then peak to peak is (positive peak - negative peak) = 1600 mV, and not
> > 800
> > mV as you have in your response. So, I agree with Mike Dudek, Ed Grivna,
> > that your statement below is confusing. I am agreeable to any way of
> > defining logic levels with or without common mode, single ended or
> > differential. I agree with Ed Grivna that Vcm is not necessary to define
> > a
> > differential signal, and I am only responding to a thread that is using
> > Vcm.
> I recently had exactly this entire argument with several of my colleauges at
> Nexabit Networks. Of course, we did not settle it, since the debate hinges
> upon the definition of the term "differential voltage swing", and semantic
> arguments can never be resolved.
> However, permit me to make one point about signal levels and swings before I
> go back to lurking: From the standpoint of the receiver of the differential
> signal, the important quantity is the voltage difference between the Vin+
> and Vin- inputs. The input stage of the receiver is almost always a diff
> amp, and it is the difference | (Vin+) - (Vin-) | = 800 mV which appears
> across the bases of the two input transistors. When a logic 1 is
> transmitted, then the diff amp sees (Vin+) - (Vin-) = +800 mV between the
> transistors' bases. When logic 0 is transmitted, the diff amp sees (Vin+) -
> (Vin-) = -800 mV. As others have pointed out, the voltage difference
> between these two logic states is 1600 mV.
> But the question is: what is the important voltage to quote, 800mV or 1600
> mV? As I see it, the phyiscally important voltage is 800 mV because this
> is the signal amplitude which much defeat any physical imperfections at the
> receiver input diff amp, e.g. input offset voltage, noise, etc. The 1600 mV
> difference between logic 1 and logic 0 is a meaningless number since the
> diff amp never sees any physical voltage of 1600 mV.
> The situation is different in single-ended logic, where the voltage
> difference between logic 1 and logic 0 is actually seen by the input
> amplifier; it is this voltage difference (e.g. 5V in old-fashoined TTL)
> which must defeat the input amp imperfections.
> > Given all this confusion, it may be prudent to include pictures of logic
> > signals, and define the swings in an already standardized way.
> Agreed. The best thing to do is draw a picture.
> Stuart Brorson
> Nexabit Networks/Lucent Technologies