Transmitter specification at 6G
Allen.N.Kramer at seagate.com
Allen.N.Kramer at seagate.com
Tue May 22 14:51:01 PDT 2007
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Once upon a time in an FC Standard long long ago we used to specify TX
Jitter as DJ, RJ, and Total jitter.
And they all added up quite neatly until one day someone made the
observation that we where over-limiting the jitter. He observed that if
one did better on one component of jitter (DJ or RJ), shouldn't he be
allowed exceed the other parameter as long as he didn't exceed Total
jitter. At the time, we had the choice of specifying "DJ and Total" or "RJ
and Total". Which ever was chosen, the opposite jitter attribute would be
allowed to exceed the limit then applied by specifying the three
components of jitter: DJ, RJ, and Total jitter. If "DJ and Total" were
chosen, receivers would have to tolerate RJ up to near the Total jitter
number if someone designed a near perfect DJ transmitter. If "RJ and
Total" were chosen, a receiver would have to tolerate DJ up to near the
Total jitter number if someone came up with a transmitter with exceedingly
low RJ. After a lot of discussion, "DJ and Total" was chosen because the
consensus at that time was that a receiver could tolerate a little more RJ
in the presence of well controlled DJ, but that well controlled RJ did not
guarantee that a receiver could tolerate DJ that was a larger proportion
of Total jitter.
The current version of 07-063 uses an "RJ and Total" jitter specification
at 6Gbp/s. While receivers have improved dramatically, I still do not
think that they have improved to the point that the transmit device's DJ
should not be specifically limited by a "DJ and Total" methodology. If
receivers can provably tolerate more DJ at the transmitter than is
currently specified, then we should increase it. However, I think it
should still be intentionally limited within TJ, leaving RJ as the jitter
component that can be increased in the presence of better DJ. After all,
we are going to be faced with closed, or near closed eyes and the primary
component of this 'closing' will be ISI, which is DJ. The ISI basically
shrinks the amplitude of high transition bits that follow a low transition
region. DJ from other sources, such as the transmitter, basically time
skew these same low amplitude high transition bits from the center of the
eye, further smearing the eye. We need not only limit the eye closure
allowed by ISI, we need also to intentionally limit the displacement of
these amplitude challenged bits from the center of the eye. The
âchannelâ
that interconnects the transmitter to the receiver creates additive DJ to
the transmit signal from such sources as ISI (mentioned above), imperfect
terminations, impedance discontinuities at connectors, viaâs, packages,
crosstalk, etc. These DJ sources also skew the low amplitude high
transition bits from the center of the eye. The effects of the majority
of these sources of DJ on a signal are not easily quantified, and the
universe of creatively intermixing these sources of DJ onto a signal in a
âchannelâ is certainly not contained in a single S-parameter file of an
external 10meter SAS cable. To then allow a transmitter without âclearly
defined DJ limitsâ to drive such a channel is ill advised. The hope of
developing a 6G standard that specifies all parts of the TX-RX link in a
manner that results in achieving the near-zero bit error rate desired by
storage system developers necessitates keeping a "DJ" jitter specification
on the transmitter.
It may even require retuning to the method of long ago: intentionally
limiting âRJâ and âDJâ, since I assume we did not change from a âDJ
and
Totalâ jitter specification (that has been used to successfully develop
several generations of near-zero bit error rate storage systems) to an âRJ
and Totalâ specification unless this change addressed a known problem.
Recovering signals that are âclosedâ is after all, a new endeavor in
near-zero bit error rate storage systems, and may require specifying both
RJ and DJ along with Total Jitter (i.e. the limit on RJ and DJ do not
necessarily have to add up to Total Jitter; the three attributes {RJ, DJ,
Total} just need to constrain the measured transmitter performance in a
manner that increases the opportunity for building a new generation of
near-zero bit error rate storage systems).
Regards,
Al
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Allen Kramer
Phone: 952-402-2624
Fax: 952-402-3471
Seagate Confidential
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