[SI-LIST] Power sum crosstalk calculation method--is there an error in OIF-CEI or 10GBase-KR?

  • From: "Qin, Zhenshui (NSN - CN/Shanghai)" <zhenshui.qin@xxxxxxx>
  • To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 29 Jul 2013 08:57:28 +0000

Hi,
I am working on an simulation for 25G operation, the results is targeted to 
meet CEI 25G LR mask.
An interesting thing I found in OIF-CEI 25G mask is the method used to power 
sum the crosstalk sources, actually the same method use also for IEEE 
10GBase-KR standard.
For example, If I have two crosstalk source, the equation should be like below:
PSXT=-10*log10(10^(-Xtk1/10)+10^(Xtk2/10))
But my question is, in SI area, our S-parameter is calculated using Voltage 
rather than Power, it means we calculate the value use
S12 *log10(Vout/Vin).
So the problem happened when we use the equation above, two -40dB crosstalk add 
up to
PSXT=-10*log10(10^(-40/10)+10^(-40/10))=-37dB
While the reasonable value should be -40dB corresponding to 0.01, and two of 
them makes 0.02, so
PSXT *log10(0.02)=-34dB

Does anyone know the trick behind the equation? Or what's wrong with my 
understanding?
Thanks.


Bruce




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