[10GBT] Validity of comparing PSDs to determine electromagnetic emissions
Scott -
Thank you for pointing out a simple fact that has made for amusing
reading while I've been busy with other things. Much of the discussion
here of PAM-12 vs. PAM-8 is well within the bounds of measurement
tolerances, and, as will be discussed below, can be flip-flopped by
design trades in these measurement tolerances. This covers more than
just EMI, so here are some of the summary points:
1) EMI emissions - We're way beyond 20 log (f) approximations. We've
seen measured results in numerous presentations by multiple sources (see
cohen_1_0903.pdf for methodology and also a presentation by Scott
Powell). Scott's right. Differences in the PSDs are TINY when compared
with differences test setups, cabling configurations, and measurement
equipment can make - if a design is so close that the small difference
in the PSD is going to make it fail, it will fail for a dozen larger
reasons. I'd be interested in hearing from Thuyen Dinh which proposal
looks easier to build magnetics for, keeping in mind both emissions and
insertion loss.
2) Coded margin - we're asking for accuracy way beyond assuming the same
coding gain, or even running AWGN simulations on non-TH Precoded
constellations. In my experience this can cloud things by 1 dB.
3) Component and noise budgeting accuracy - The solarsep simulation code
with Salz-bounds provides an optimal DFE basis which can be used as a
line/environment limited bound on how well you can do. It does NOT
provide a basis for evaluating sensitivity to noise which enters the
receiver at different points in the receive chain, hence it should not
be used by varying the AWGN level to try to measure sensitivity to
receiver noise, or for accurately tweaking the transmit and receive
filtering. It can be adjusted for that, but for that you need to become
architecture-specific to a particular vendor, and add additional linear
transfer functions (such as magnetics, location of gain stages, sources
of noise and hybrid structure). Net-net, the degradation from optimal
becomes significantly vendor-architecture dependent and therefore less
capable of multiple vendors tracking the results.
4) EMI susceptibility - we're also way beyond looking at susceptibility
just as the partition separation of a sine wave on a single pair in the
absence of the consider ANEXT noise (e.g., Crane's test), because in the
presence of noise the cumulative coded errors will cause errors first.
Real EMI happens on all 4 pairs at once (to differing levels), generally
in the presence of noise. In this situation, generally, the code breaks
down way before the partition difference (at least for 1e-12 BER). As
such, the EMI susceptibility must be measured in the presence of full
ANEXT. This will require LDPC code simulations in the presence of EMI
ingress and ANEXT. It must also respect the difference in receive gain
required by the PAM 12 and PAM 8 systems. The answer is not obvious,
but I'm pretty sure I haven't seen it discussed on this forum amidst all
of the heated arguments, AND, it really requires selection of a code to
get it done.
Long and short -
We need to get this matter decided already. However, in order to
finalize this decision, we need not only PAM levels on the table, but
code and modulation rates nailed. We also need to consider impact on
non-silicon components (cabling and magnetics, both of which are
stressed for bandwidth). There are only a small number of points that
have been made that I think are unambiguous:
1) The PAM-8 baud rate is faster, so for a fixed cancellation ratio, it
will require more digital processing. If it, in fact, has higher
margins, these may be traded for implementation loss to ease the burden.
2) The PAM-12 constellation with a hole in it is inefficient. This fact
is responsible for more than one of Sailesh's complaints. The cross
constellation just moves the inefficiency around to a more familiar
form, but still suffers precoding losses. A more efficient mapping can
be adapted.
3) The framing proposed in the PAM-12 is complex and possibly can be
improved.
4) The choice of transmit filter will govern the emissions more than the
number of PAM levels.
5) The sensitivity of margin to baud rate is broad enough that the
ANEXT-limited performance differences in an optimal signal processing
sense are small.
-george zimmerman
---
-----Original Message-----
From: stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On
Behalf Of Scott Powell
Sent: Tuesday, August 31, 2004 11:29 AM
To: STDS-802-3-10GBT@LISTSERV.IEEE.ORG
Subject: Re: [10GBT] Validity of comparing PSDs to determine
electromagnetic emissions
I agree with Kishore's point that there is not a significant EMI
difference
between the two proposals. It's been shown a couple of times on the
reflector that one proposal is slightly better over certain frequencies
and
the same proposal slightly worse over others. In response to a recent
posting on the importance of "slight" PSD differences:
> "saliesh rao" wrote
>
> With regard to issue #1, I disagree that a 1dB difference in the
transmit
PSD is "well below the
> measurements and setup errors" for the emissions profile. Anyone who
has
spent hours and days
> meeting the FCC/CISPR emissions profile in the range will know that
1dB
will make a huge difference
> between a pass and fail, since the likelihood of a fail would be
exponentially determined by this single dB.
>
I would respectfully request that you take a look at ANSI standard C63.4
before relying too heavily on the fractional dB emissions you think you
are
measuring. This standard document can be ordered from the IEEE and
describes the measurement accuracy that certified EMI test ranges must
comply to. As you can see in section 5.4.6.1 on page 20, the
"Acceptability
Criterion for Emission Test Range" is +/- 4dB. Your EMI range may not
always "volunteer" this information but should if you ask.
ANSI C63.4 standard means that if the same 10GBASE-T transceiver (+
cabling/connectors) is measured in 2 different *ANSI Qualified* EMI test
ranges and results differ but are within +/- 4dB of each other, the
measurements are both considered valid - even though they differ by +/-
4dB.
Susceptibility accuracy requirements for qualified ranges are even
worse.
These permitted inaccuracies are in addition to the inherent set-up
inaccuracies of finding the "worst case" antenna height, antenna
polarity,
and cable configuration/orientation for each frequency in the scan.
I don't believe the small PSD differences shown so far between the two
proposals are significant enough to warrant choosing one over the other
based on expected emissions. Again repeating my original message: "it
has
been clearly shown that the PAM-8 and PAM-12 proposals have similar
performance (slight edge for PAM-12), but the PAM-12 proposal offers the
advantage of a reduced operating frequency resulting in reduced power
and
reduced implementation difficulties."
Regards,
- Scott
-----Original Message-----
From: stds-802-3-10gbt@ieee.org [mailto:stds-802-3-10gbt@ieee.org] On
Behalf
Of Kishore Kota
Sent: Tuesday, August 31, 2004 7:21 AM
To: STDS-802-3-10GBT@listserv.ieee.org
Subject: [10GBT] Validity of comparing PSDs to determine electromagnetic
emissions
I would like to question the validity of comparing the power spectra
(PSD)
of various proposals to determine electromagnetic emissions. In earlier
postings on this reflector, such comparisons have been used to claim
superior emissions performance for PAM8. There are several unknowns
here:
a) As a taskforce we haven't agreed to a model for how differential
signal
gets converted to common mode, and
b) we do not have a model for how the common mode signal then gets
converted
to electromagnetic emissions.
Simply comparing PSDs without applying such models would lead to
nonsensical
conclusions. For instance, in the lower frequency band 1000BASE-T
spectrum
is atleast 8db higher than either of the 10GBT proposals (because the
same
transmit power is spread over a smaller frequency band). However, this
does
not mean 1000BASE-T is tougher than 10GBT for emissions compliance
testing.
Some comparisons of PAM12 and PAM8 spectra have been posted to the
reflector
which include a 20logf model. Although this is a crude approximation, it
does seem to have some consensus in the group. By playing games with
this
model and lowpass filters, either proposal can be made to look slightly
better than the other. This suggests that there are no significant
differences between these proposals as far as emissions are concerned.
Obsessing over these differences just keeps us from answering the more
difficult question of whether either proposal is sufficient from an EMI
perspective.
regards,
Kishore