[SI-LIST] Re: Definition of "crosstalk loss" ??
- From: agathon <hreidmarkailen@xxxxxxxxx>
- To: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- Date: Wed, 2 Jan 2008 01:34:46 -0800
Yes, thanks, Scott and Richard. I realized after posting my inquiry that
it's just like the other "losses" but not due to the victim
conductor-connected sources; rather, the aggressor.
But, Scott, the loss is in the f domain, so any "lining up" you mention is
purely an unrelated time domain issue.
The ieee 802.3ap Annex 69B pwr sum is defined there as:
PSFEXT = -10 log SUM (10^-[FEXTn(f)/10]) , a linear sum of Sdd32 for each
n, expressed in dB. Ie, PS is sum then power. n = aggressor index.
-Agathon
On Dec 5, 2007 11:04 AM, Scott McMorrow <scott@xxxxxxxxxxxxx> wrote:
> Oops
>
> that should have been:
>
> Sdd21 = insertion loss of victim
> Sdd43 = insertion loss of aggressor
> Sdd13 = near end crosstalk NEXT
> Sdd23 = far end crosstalk FEXT = "crosstalk loss"
>
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
> http://www.teraspeed.com
>
> Teraspeed(R) is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
>
> Scott McMorrow wrote:
>
> Actually, I believe all measurements are made w.r.t. the victim.
>
> Crosstalk loss is the energy that the sum total of the aggressors impart
> on the victim. In a 4-line system with 4 differential ports.
>
> Sdd21 = insertion loss of victim
> Sdd43 = insertion loss of aggressor
> Sdd13 = near end crosstalk NEXT
> Sdd21 = far end crosstalk FEXT = "crosstalk loss"
>
> When the moon and stars line up and all aggressors fire at a point and
> phase where they destructively interfere with the victim at the
> receiver, then you have an apparent loss, or "crosstalk loss".
> Interestingly enough, if the aggressors constructively interfere with
> the victim, then you get an apparent gain, or "crosstalk gain". They
> use a power sum for multiple aggressors, based on the assumption that
> not all aggressors will be at the worst case phase alignment with the
> victim at any time. Personally, I'd much rather use a linear sum, just
> to be sure.
>
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
> http://www.teraspeed.com
>
> Teraspeed(R) is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
> wolfgang.maichen@xxxxxxxxxxxx wrote:
>
>
> I have to admit I haven't heard that term myself before. But from the text
> it looks like they mean the signal loss in the agressor - keep in mind
> that the crosstalk signal (energy) in the victim has to come from
> somewhere, so if there is to be a crosstalk signal on the victim, then the
> agressor must experience signal loss. Loss in aggressor signal voltage is
> often not as easily visible, for two reasons. Since power is proportional
> to the square of the voltage, a 1% energy loss due to crosstalk amounts to
> only an approx. 0.5% reduction in aggressor signal. In addition, it's
> easier to observe the crosstalk on the victim going from e.g. 0 to 10mV -
> a large relative change - than it is to observe the aggressor signal going
> from 1000mV to 995mV - a very small relative change -, which can hide the
> effect in practical scope measurements and lead to the false belief that
> crosstalk only affects the victim but not the aggressor.
> Wolfgang
>
>
>
>
>
>
> agathon <hreidmarkailen@xxxxxxxxx> <hreidmarkailen@xxxxxxxxx>
> Sent by: si-list-bounce@xxxxxxxxxxxxx
> 12/04/2007 05:01 PM
> Please respond tohreidmarkailen@xxxxxxxxx
>
>
> To
> si-list <si-list@xxxxxxxxxxxxx> <si-list@xxxxxxxxxxxxx>
> cc
>
> Subject
> [SI-LIST] Definition of "crosstalk loss" ??
>
>
>
>
>
>
> Hello all,
> I'm trying to make clearer the following statement from 802.3ap:
>
> "...FEXTn is the crosstalk loss, in dB, of aggressor n."
>
> What the heck is "crosstalk loss"????
>
>
> The full paragraph is:
> 69B.4.6.2 Power sum differential far-end crosstalk (PSFEXT)
> The differential far-end crosstalk at TP4 is calculated as the power sum
> of
> the individual FEXT aggressors
> (PSFEXT). PSFEXT is computed as shown in Equation (69B?16), where FEXTn is
> the crosstalk loss, in dB,
> of aggressor n. Note that for the case of a single aggressor, PSFEXT will
> be
> the crosstalk loss for that single
> aggressor.
>
>
> Thanks,
> Agathon
>
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