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From talking to several 10GBase-T EMI experts, the general consensus is that EMI emissions increase with frequency as 20*log10(f).
Thus the system with the higher symbol rate (PAM8 at 1000MHz) will have worse EMI if the tx power is the same and both have equivalent digital and analog filtering.
The plot below shows the EMI PSD for 1GBase-T, PAM12 and PAM8 assuming all have the same digital filter 3/4+1/4D. As expected the PAM8 is the worse, since it has the most signal at higher frequencies.
I only plotted the PSD up to 0.4*SymbolRate, because the analog filters will typically have >3dB attenuation at 0.5*Symbol rate and the PSD is mostly dependent on the analog filters. PAM12 and PAM8 will both have similar analog filters.
For those interested in the details I included the matlab code. For a fixed tx power the PSD level (dBm/Hz) will drop as -10*log10(BW) when we increase the bandwidth, BW. But as we increase the BW, the EMI increases as 20*log10(BW). Clearly the increase in EMI in PAM8 is twice as bad as the PSD level drop.
Thus PAM12 has 10*log10(1000)=0.8dB more signal at low frequencies, but PAM8 has 1.6dB more EMI at high frequencies. Therefore PAM8 has 0.8dB more EMI
From: stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On Behalf Of sailesh rao
Sent: Tuesday, July 20, 2004 6:55 PM
Subject: [10GBT] Request for Cat6 Emissions Characteristics
At the request of a task force member, I computed the transmit PSD for PAM8,
PAM12 and 1000BASE-T, and discovered that I added the contribution of Jose's 0.4dB incorrectly in my Total EMI Penalty calculations in the updated table.
Actually, in the passband, the transmit PSD for PAM8 after THP is 1.6dB lower than the transmit PSD for PAM12 after THP, and not 0.8dB lower as I had reported earlier.
However, rather than simply adding another 0.8dB to the PAM12 EMI penalty, I would like to propose that we compute the estimated emissions characteristics of the PAM8/PAM12 transmit PSDs, for different transmit filtering choices, over typical Cat6 cabling and assess the difference in peaks to estimate the true emissions penalty for one PAM approach over the other. I know Scott Powell had published a PDF version of measured 4-connector Cat6 emissions characteristics for a flat transmit PSD (powell_1_0303.pdf), but do cabling experts agree with Scott's measurements and can the task force use that data to assess the emissions characteristics of PAM8 vs. PAM12? If so, I would like to request Scott to publish a file version of his measurements on the task force web site so that we can all use it to do fair comparisons. If not, are there other typical Cat6 cabling emissions measurements that we can use to do the emissions comparisons?
Any input from the cabling experts on the reflector would be greatly appreciated.
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