[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.