[SI-LIST] Crosstalk, part deux

After my recent message about using spectral information to track down =
crosstalk, I was reminded that there is an even simpler and more direct =
technique to track down crosstalk offenders with an oscilloscope. This =
technique also is limited to crosstalk that is uncorrelated to the =
signal.=20

Assume for example you have a 33 KHz switching power supply that you =
suspect is coupling into another signal, causing crosstalk. Connect one =
channel of the scope (or its external trigger input) to the 33 KHz =
switching power supply and trigger the scope on it. Then view the victim =
signal on another scope channel. Turn on averaging in the scope. All the =
components of the signal that are not correlated to the 33 KHz signal =
will be suppressed by averaging. This will allow you to see to what =
extent the coupling is affecting the signal. In this analysis we treat =
the desired signal as "noise" and the crosstalk signal as the "signal." =
Suppressing the "noise" (the intended signal) reveals the "signal" (the =
crosstalk).  =20

Unfortunately this doesn't work (or at best gives ambiguous results) for =
crosstalk that is correlated to the victim signal, as is often the case =
in digital circuits. Most switching in digital circuits is synchronous =
to the same clock (or multiples or submultiples of the same clock) so =
there is often a high degree of correlation between aggressors and =
victims.=20

Art Porter     =20
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