Receiver JTF tolerance

Bent Hessen-Schmidt bhessen at SynthesysResearch.com
Tue Jul 22 10:40:46 PDT 2008


Formatted message: <A HREF="r0807221_f.htm">HTML-formatted message</A>

Hi Guillaume,
4600 ppm peak-to-peak (45.3 UI) of sinusoidal modulation at 97 kHz and
1840 ppm pp (6.5 UI) at 270 kHz are OK. 
I think that it is important that we have these two test points on the
sloping side of the Jitter Transfer Function (JTF). The section in
between is theoretically a straight line.
Some people had understood that we wanted to test at 10 kHz or with
14870 ppm pp at 30 kHz and I think that we have to clarify that these
two points exceed the expected frequency deviation range of the receiver
VCO, whereas 97 kHz was selected to be the point where the receiver CDR
should be able to handle deviation as well as jitter tolerance. 
Sincerely,
Bent Hessen-Schmidt
SyntheSys Research, Inc.
3475-D Edison Way
Menlo Park, CA 94025
(650) 364-1853
www.BERTScope.com
________________________________
From: Guillaume Fortin (Montreal)
[mailto:Guillaume_Fortin at pmc-sierra.com] 
Sent: Monday, July 21, 2008 12:43 PM
To: Bent Hessen-Schmidt; t10 at t10.org
Subject: RE: Receiver JTF tolerance
Hi Bent, 
I agree that the modulation at 97kHz that we will be applying is likely
higher than what would be applied in a reasonable SAS-2 system with
30kHz triangular SSC modulation. However, the residual jitter that
results from the 45.3UIpp modulation at 97kHz creates a 0.1 UIpp
residual jitter after the JTF and thus meets the SAS-2 specification,
that is to say that a compliant transmitter could generate it. If the
transmitter is allowed to generate this jitter, we must ensure that the
receiver can track it.
By ensuring that a compliant transmitter never produces more than 0.1UI
of residual jitter after the JTF and ensuring that a receiver accepts an
input jitter that is at least 0.1UI after filtering by the same JTF, we
ensure consistency in our analysis of system robustness. This is what
the inverse JTF mask accomplishes.
A 4600ppm-pp sine SSC modulation would meet the spec so I disagree with
the 1.3x factor for triangular vs sinusoidal. I am not sure that I
follow your train of thoughts with the modulation index. It would seem
to me that the modulation index (m = df / fm) would actually decrease by
a factor of 3.23 at 97kHz vs 30kHz since the modulation frequency
increases (30kHz -> 97 kHz) while the df stays constant (2300ppm * 6GHz
= 13.8MHz).
Regards,
Guillaume
________________________________
From: Bent Hessen-Schmidt [mailto:bhessen at SynthesysResearch.com] 
Sent: Monday, July 21, 2008 1:16 PM
To: Guillaume Fortin (Montreal); t10 at t10.org
Subject: RE: Receiver JTF tolerance
Hi Guillaume,
The +/-2300 ppm (4600 ppm peak to peak) deviation may be the same for
your proposal, but please realize that the modulation index will be 3.23
times larger at 97 kHz than it is at 30 kHz. Further the sinusoidal
component of a 5000 ppm peak-to-peak triangular SSC waveform is merely
3626 ppm at 30 kHz (another factor of 1.3 of difference). 
Your proposal therefore is about 4.2 x the modulation which can be
generated at 97 kHz in SAS system. Instead a factor 2 x, which is very
close to your peak (not peak-to-peak) values in ppm as displayed in your
graph, will be much more reasonable for receiver testing. 
Bent
________________________________
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of
Guillaume Fortin (Montreal)
Sent: Monday, July 21, 2008 8:29 AM
To: Bent Hessen-Schmidt; t10 at t10.org
Subject: RE: Receiver JTF tolerance
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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