Re: XAUI skew
Tord and others -
Below are words that Fibre channel (see FC-PI, revision 11) uses for skew. I think it is a good definition, in that jitter, rise/fall times, etc. may be present, but don't confound the measurement. The "test load" would be 100 Ohms +/-1%.
"A skew measurement is valid only for balanced driver configurations. The measurement is to be made at the sink side of mated connector pairs, and across a load equivalent to those shown in figure 41, ‘Test loads,’ on page 71. These are single-ended measurements and assume a.c. coupling between the oscilloscope and the driver. A valid pattern such as CRPAT or primitive sequence should be transmitted during this test.
For each signal (true and complement), measure the mean of the eye crossing using a horizontal histogram vertically centered at the average value of the waveform. The same stable trigger, coherent to the data stream, shall be used for both the true and complement signals. Skew is defined as the time difference between the two means."
Hope this helps.
Tom Lindsay
Vixel
425/806-4074
Tord Haulin wrote:
> Dawson,
>
> Clause 47 has specifications on differential skew both for the
> transmitter and receiver. There is however no definition for
> differential skew, or method given for measuring it. In paragraph
> 47.3.4.1 the differential skew is said to include the effects of
> intersymbol interference, ISI. I would think the main effect of ISI is
> deterministic jitter, which is part of the jitter specifications. To
> define and measure skew, we would probably have to relate it to rise and
> fall times and define a common mode load and load balance with
> tolerances. The transmitter skew and possible crosstalk along the line
> is filtered by the channel common mode transmission magnitude and group
> delay, which falls outside the channel model work done to date.
>
> In the receiver compliance test some additional sinusoidal jitter is
> applied to ensure some margin for effects otherwise not accounted for.
> How about lumping skew into this, to avoid going into specifying enough
> parameters and conditions to take measurements on skew and related
> parameters? That would mean the skew limits could be removed from the
> spec. Comments?
>
>
>
> Tord.
>
>
>
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