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 18.104.22.168 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."
From: email@example.com [mailto:firstname.lastname@example.org] On Behalf
Of Kishore Kota
Sent: Tuesday, August 31, 2004 7:21 AM
Subject: [10GBT] Validity of comparing PSDs to determine electromagnetic
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