Comment on 08-032r2, SSC Profile definition

Guillaume Fortin (Montreal) Guillaume_Fortin at pmc-sierra.com
Fri Jan 11 10:50:22 PST 2008


* From the T10 Reflector (t10 at t10.org), posted by:
* "Guillaume Fortin (Montreal)" <Guillaume_Fortin at pmc-sierra.com>
*
I agree that your alternative text is clearer.
As a note, the original text was intended to refer to the averaging the slope
(df(t)/dt), which would be of the form: integral from t-tau to t of df(t)/dt
divided by tau. This is mathematically equivalent to the simpler
(f(t)-f(tau))/tau that you propose.
I will publish and r3 with your changes later today.
Regards,
Guillaume
-----Original Message-----
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Chuck Hill
Sent: Friday, January 11, 2008 11:01 AM
To: t10 at t10.org
Subject: Comment on 08-032r2, SSC Profile definition
* From the T10 Reflector (t10 at t10.org), posted by:
* Chuck Hill <cphill at altaeng.com>
*
All,
08-032r2 proposes the following language in section 5.3.8.1: "The slope of
the frequency deviation shall not exceed 1200 ppm/µs when averaged over any
0.3µs (±0.01 µs) window of the SSC modulation profile."  This is confusing
and imprecise. 
The input waveform (the SSC profile) is a time waveform with value
corresponding to frequency and units of ppm.  Call this waveform f(t).	
The slope is computed by f(t)-f(t-tau) / K  where K is a constant factor, the
separation in time (for SAS K=0.3uS).  This is a simple difference equation
applied to the waveform that is not a low pass filter.	An averaging process
is of the form: Integral from t to t-tau of f(t) and has a strictly falling
response as frequency increases, a low pass filter. 
The language of 08-032r2 leads the reader to believe the slope of the
frequency deviation is averaged, thus low pass filtered, and that is not the
intention.  It also does not precisely define the required computation as
there is a time separation tau, as well as a scaling factor K.
I offer an alternative text to replace the previously mentioned
sentence:  "The slope of the frequency deviation shall not exceed 1200 ppm/µs
when computed over any 0.3µs (±0.01 µs) interval of the SSC modulation
profile.  The slope is computed from the difference equation
f(t)-f(t-0.3uS) / 0.3uS where f(t) is the SSC profile expressed in ppm."
Regards,
Chuck
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