Removing

Gourgen Oganessyan gourgen at quellan.com
Wed Jul 16 22:47:46 PDT 2008


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While I agree that there may be an inconsistency between Table 61 and
Figure 125 (numbering changed in v14c), I am not sure I agree with the
remedy. Forcing the 30 mVpp RMS value across the board will impose a
high 84.85 mVpp TX common mode voltage limit at low frequencies < 300
MHz. At those frequencies the CM noise will not get attenuated to any
appreciable degree by the channel, so the receiver will be hit with a
large part of that, leaving very little margin out of the 150 mVpp RX
common-mode tolerance to account for other sources of common-mode noise
injection along the channel. I would be OK with revising the number in
Table 61 to correctly account for Fig 125, or else keep the 100-300 MHz
range in Fig 125 a low 12.2 mVpp flat line and going to a high 84.85
mVpp value after that. 
Gourgen Oganessyan
Quellan Inc.
Phone: (630)-802-0574 (cell)
Fax:	 (630)-364-5724
e-mail: gourgen at quellan.com  
________________________________
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Seidel,
Mark N
Sent: Tuesday, July 15, 2008 12:49 PM
To: t10 at t10.org
Subject: Removing 
T10 Phy WG,
The transmitter common-mode voltage limit is specified in two places,
once in Table 61 as a broadband limit, and again in Figure 123 as a
per-frequency-band limit.  Table 61 limits the overall amount to 30
mVrms which translates to 84.9 mVpp if the AC signal is a pure sinusoid.
Figure 123 imposes a limit in the band 100 MHz to 300 MHz (1 MHz
measurement band) to 12.7 dBmV which translates to 4.3 mVrms and 12.2
mVpp if it is a pure sinusoid.
Furthermore, if the transmitter had energy at each band that followed
the limits in Figure 123 it would far exceed the limit in Table 61.  A
collection of discrete frequencies at the limit in Figure 123 would
violate the overall limit, such as (for example) spikes at 100 MHz, 200
MHz, and so on up to 1400 MHz at the Fig 123 limit would violate the
overall limit.	As another example, spikes at 100 MHz, 300 MHz, 500 MHz,
and so on up to 1900 MHz would violate the limit, as would a set of
spikes at 750 MHz, 1500 MHz, 2250 MHz and 3000 MHz.   Note that these
spikes would violate the "energy" aspect of the limit, where they are
combined as a sum-of-squares and then translated effectively into a
sinusoid.  The actual combination of the spikes would depend on the
relative phases; the smallest I could find for the 750/1500/2250/3000
MHz case was approximately 40 mV, so that combination of frequencies
could not all simultaneously be at the Fig 123 levels.
Since these sinusoidal levels are quite small, I propose that we remove
Figure 123 and its limits entirely and retain only the limit in Table
61.  This absolute wide-band time-domain specification will be enough to
limit transmitted CM energy and still allow the silicon and system
designers enough leeway to specify their power supply noise and
filtering limits depending upon the particular frequencies in their
systems.  This leeway should not jeopardize practical systems.
Mark Seidel
Principal Engineer
Intel Corporation
Chandler, AZ



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