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Re: XAUI Jitter Agenda 28th Febuary 2001




Gentlemen,

From the teleconference this morning, I owed a response on the issue
of AC coupling in the standard 100 ohm load, in particular, the 
pole frequency of the AC coupling.  For passing 8B/10B coded data,
table B.1, "Fibre Channel - Methodologies for Jitter Specification"
a ratio of pole frequency to bit rate less than about 0.0002
produces insignificant jitter (due to the capacitor's non-zero
impedance).  Hence, pole frequency < 0.0002*3125 MHz = 0.625 MHz.
For my money, 1 MHz is close enough.  The smallest value blocking 
capacitors I've seen in applications is 2400 pF, which, on a 50 ohm
line, yield a 1.326 MHz pole.  Likewise, 0.01 uF yields 0.318 MHz.

One other effect to consider is that (assuming the measurement is 
taken after the blocking capacitor), while the capacitor's
non-zero impedance introduces jitter, it acts as a high-pass filter.
Therefore it actually REDUCES the jitter created by the channel.  
Hence, the pole frequency should be restricted from being too high
to preclude accidentally or intentionally optimistic measurements.

In summary, I think 1 MHz is a good value, permitting use of
reasonably valued capacitors and minimizing jitter induced by 
the coupling capacitor itself.

Regards,
Mike
-- 
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 Mike Jenkins               Phone: 408.433.7901            _____     
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