RE: [SI-LIST] : Far end crosstalk

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From: Issa, Elie (
Date: Fri Oct 13 2000 - 09:35:59 PDT

<I guess my question would be, why does it have to be a ratio of anything?>
Because CrossTalk is characterized as a Gain for the quite line. So it has
to be a ratio.
Since it is a voltage gain, it should be compared to the input voltage of
the aggressor
 The voltage at the far end of the aggressor would include losses unrelated
to cross talk which you are not interested in this discussion
I believe the definition of the far end cross talk would be, FEXT = Voltage
at the far end of the quite line / Vin of the aggressor line.
You need to note that some time when you are trying access your device, you
may need some fixturing. such fixturing would degrade & slow your rise time

and attenuate the input voltage from the source.
You need to characterize those two quantities when you are reporting your
CrossTalk or shopping for a product with good crosstalk performance. The
attenuated amplitude from the output of the fixture would need to be in
calculation of Crosstalk as the input aggressor voltage and not the source
voltage of the instrument . Also you would be interested to know the Rise
Time(since cross talk is dependent on it). Here also should be the measured
Rise Time from the output of the fixture and not the RiseTime of the
instrument. Not to consider those factors would make CrossTalk (far end or
Near end) look better.

Elie Michael Issa
Signal Integrity Group
Corporate Design Automation
ISSD, Enterprise Computing Group
COMPAQ Computer Corporation, Houston
Phone: 281-518-5492 / Fax: 281-518-0652


-----Original Message-----
From: Peterson, James F (FL51) []
Sent: Friday, October 13, 2000 10:00 AM
Subject: RE: [SI-LIST] : Far end crosstalk

Hello Ohio,
I guess my question would be, why does it have to be a ratio of anything?
Why can't it just be the amount of noise visible at the far end of the
victim's transmission line?
I guess if it is a rating of a t-line's capacity for crosstalk then you
might go with a ratio (for x amount of voltage injected at the aggressor's
t-line you will get x mV of coupled noise at the victim's far end). If this
is the case I would think you would use the aggressor's initial signal

-----Original Message-----
Sent: Wednesday, October 11, 2000 12:48 PM
Subject: [SI-LIST] : Far end crosstalk

Greetings from Ohio.
What is the correct definition for far end crosstalk for a transmission line
(cable): is it the ratio of the voltage measured at the far end of a quiet
line vs. the input voltage on the active line or vs. the output (far end)
voltage on the active line, which is an attenuated version of the input

FEXT = Vout quiet/Vin active or Vout quiet/Vout active?

John F. Sawdy, Senior Applications Engineer
Meritec, a division of Associated Enterprises
voice mail: 440-354-3148 x 267
fax: 440 -354-0687 <>

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