Receiver JTF tolerance

Guillaume Fortin (Montreal) Guillaume_Fortin at pmc-sierra.com
Mon Jul 21 08:29:16 PDT 2008


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You are correct Bent: the frequency offset values are peak values.
It is true that the frequency offset at 30kHz is too large if we scale
the SJ modulation by the inverse-JTF below ~2MHz. This is why my
proposal is to stop at 97kHz, which is the point at which the SJ
modulation amounts to +/-2300ppm, which is the maximum SSC modulation
that has to be tracked according to the SAS-2 spec.
I will create figures for the SJ mask that should make things clear.
Regards,
Guillaume
________________________________
From: Bent Hessen-Schmidt [mailto:bhessen at SynthesysResearch.com] 
Sent: Friday, July 18, 2008 2:40 PM
To: t10 at t10.org; Guillaume Fortin (Montreal)
Subject: Receiver JTF tolerance
Hi Guillaume,
There appears to be an error going from the equation for frequency
offset on page 9 to the figures on page 10 and forward in your
presentation document 08-248r0. When the offset plotted is only half of
the peak-to-peak offset as the max and min values of Cos(x) are 1 and -1
respectively and the peak-to-peak frequency offset therefore should be
double the values plotted. i.e 14870 ppm at 30 kHz for 6 Gb/s.
These are extremely large values considering that the peak-to-peak
transmitter deviation is 5000 ppm plus/minus frequency tolerance. The
transmitter SSC waveform is often a triangular waveform in which the 30
kHz sinusoidal frequency component is merely 3624 ppm peak-to-peak
followed by a decreasing amount at each odd harmonic of 30 kHz.   
I do not see any source of ppm amplification in our SAS channel. We
should certainly make sure that the receivers have some margin (i.e.
that the SSC Waveform becomes a small or insignificant portion of our
jitter budget). 4x margin may however a too much margin. We therefore
suggest that the margin be 2x instead of 4x, this will leave the current
ppm frequency offset numbers as is in the graphs while the UI numbers be
half of the values of your presentation.
Sincerely,
Bent Hessen-Schmidt
SyntheSys Research, Inc.
3475-D Edison Way
Menlo Park, CA 94025
(650) 364-1853
www.BERTScope.com



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