[802.3ae_Serial] From serial PMD call 11 Dec
More food for thought. Progress comes in very small steps - here, we may
have clarified the meaning of DCD and the need for a lone bit pattern - but
we have far too little test results to base firm decisions on. We seem to
be many weeks away from "technical feasibility" of the test procedures, and
without that we can't finalise spec numbers such as sensitivity or jitter.
Creating stressed eye
---------------------
Steve reported that with coax cable, he achieved the right amount of ISI
with about 12 ps DCD, more than he needed. If this is generally the case,
could use a lesser amount of coax cable to create the DCD then a filter to
increase the ISI.
However, one may get more practical and repeatable measurements by putting
the ISI in one test and the jitter in a different one.
To create a slow but not jittered eye, filters in the range 3-4 GHz may be
required. Unusually, we would be using such a filter with frequencies much
above the filter's bandwidth, and the group delay may not be well controlled
above say twice the bandwidth. Bad group delay could introduce jitter
again. Cascading two or three 7.5 GHz filters might create ISI and pulse
shrinkage in the right ballpark - would anyone like to try?
Measurement
-----------
Can we make more use of the scope rather than the BERT, as it may have less
distortion?
Can we make more use of the algorithms built into the scope? Can we use the
"% in +/-1,2,3sigma" readout?
Lone bit pattern
----------------
It appears that to measure ISI in the stressed eye we haven't found an
alternative to the "lone bit pattern" concept, but the
64-zeroes,1,0,64-ones,0,1..... pattern has an extremely low transition
density. While this might not be a fundamental problem, we do show a golden
PLL in fig. 52-13 so a more reasonable pattern is desired. A pattern
length which is a multiple of 66 bits may be desired.
One approach is to introduce 1010... sequences to raise the transition
density. Another is to shorten the runs, e.g. 000010111101 repeated 11
times is 132 bits (2, 66 bit blocks) long). Can we measure A_N and A_O of
fig. 52-14 well enough with either of these? Do we get acceptable
calibration by calibrating with a lone bit pattern and measuring with a
mixed frequency pattern?
Metric for signal amplitude
---------------------------
Our metric is OMA on a square wave, equivalent to A_N in fig. 52-14.
Oscilloscopes may not measure well on square waves [After the meeting: there
is "V amplitude" = Vtop = Vbase where Vtop|base is the most prevalent value
in the upper|lower portion of a pulse. Needs repetitive signal?]. They
have two algorithms built in: "Eye amplitude" which is the difference
between the mean one and mean zero in the center 20% in time of an eye
diagram, and "Eye height" which is
(mean "one" - 3.sigma) - (mean "zero" + 3.sigma). The latter is too
pessimistic where ISI is present. Other algorithms could be used, if we
knew what we wanted.
Jitter calibration
------------------
We still cannot calibrate the jitter bathtub from the BERT. One or other of
the pattern generator and the error detector adds significant data dependent
jitter. This cannot be calibrated out in any convenient general way like
root-sum-of-squares because it could combine constructively or destructively
with the data dependent jitter of the device under test. One hypothetical
way forward would be to never try to measure "W" in a calibrated way, but
apply SJ to the desired W. I suppose this is useful when creating the
stressed eye if we think that the "W" from the pattern generator is
tolerable; this way we would avoid the "W" in the error detector. [Further
thoughts while writing these notes: But we still need to calibrate the
transmitter under test jitter bathtub measurement, or drop the "shall" from
this test. Assuming the pattern generator has no "W" and calibrating to SJ
seems too optimistic.]
Discussion of Juergen's jitter measurements
-------------------------------------------
See email thread. Juergen gave some ballpark estimates of ageing margin in
jitter terms: e.g. 0.1 UI margin, or where SONET/SDH require 0.15 UI jitter
tolerance, expect to see 0.3 in the lab. He hopes to continue his jitter
measurements.
What does DCD mean?
-------------------
DCD stand for duty cycle distortion. More precisely, we have two competing
definitions:
A. The average pulse shrinkage of isolated ones, or pulse lengthening
of isolated zeroes. This can be determined with an oscilloscope, either
with a square wave (not 1010 - too pessimistic) or on an eye diagram, by
comparing the average time at which the rising edges cross 50% and the
average time at which the falling edges cross 50%. As the position of the
edges is pattern dependent, these two measurements would give slightly
different results.
B. Twice the amount by which the shortest isolated symbol in a sequence
is shorter than the nominal symbol period. This could be determined with a
time interval analyser or a scope triggered on the pattern.
In either case we don't seem to care whether it is one or zero which has
shrunk, and we quote a positive number.
Fibre Channel follow definition A. My scope instructions describe the eye
diagram method, and also a "duty cycle" measurement for a square wave. In
Gigabit Ethernet, and considering the way the Gigabit link model works, it
appears that definition B is more relevant, at least when using the 8B10B
code [separately, Jonathan pointed out that type A distortion will affect
the receiver's slicing level].
Petar suggested we should consult the official IEEE dictionary (IEEE 100,
The Authoritative Dictionary of IEEE Standards Terms) which is not on line (
http://standards.ieee.org/faqs/Std100.html ). Dear reader, if you have this
book, is Duty Cycle Distortion defined there?
On the call it was felt that 6 ps of average shrinkage would be excessive.
If that is the consensus view, we could keep the 6 ps, if that's a
reasonable amount, and change the name to "instantaneous pulse shrinkage" or
similar. Or, we could keep the name, use definition A, and reduce the
amount, probably to an insignificant amount so that we would not have to
create it in the stressed eye generator. The prospect of managing both
types of pulse shrinkage was viewed with disfavour; things are
over-complicated already and we were having this discussion with a view to
simplification!
Upper cutoff frequency test
---------------------------
Petar's colleagues are working towards the receiver upper cutoff frequency
test, but couldn't report feasibility (or lack of) yet.
Status of golden PLL
--------------------
Availability of clock recovery unit for "golden PLL" use. There should be
one soon, with 4-5 MHz jitter transfer bandwidth. We don't yet know how
much jitter it will generate (or filter out). Clock recovery units can
create pattern-dependent jitter.
Next phone meeting
------------------
The next PMD teleconference is tomorrow at the usual time and coordinates:
4:15 pm GMT = 17:15 CET = 11:15 am EST = 8:15 am PST, Tuesday
+1(816)650-0631 Access code 39209
The main topic will be optical testing again. Note this will be the last
teleconference before the holiday season. The series can restart on 8
January, just a few days before the ballot closes and the next face-to-face
meeting.
Piers