Comment on 08-032r2, SSC Profile definition

Chuck Hill cphill at altaeng.com
Fri Jan 11 08:00:48 PST 2008


* 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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