Recently I've read some documents about cables impedence that says that external EMI may create noise where there's an impedance mismatch. In the case of PCBs, with signals that lay on a solid ground layer, how much I should worry about this EMI if I've a small impedance mismatch that doesn't harm my signal quality? Is it something that I should care or this applies only on cables?
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2Are you referring to impedance mismatch in twisted pair cable - please provide a link to what you have read so that the question has some context. Possible related question and answer: http://electronics.stackexchange.com/questions/90358/terminating-shielded-screened-twisted-pair-correctly – Andy aka Nov 07 '16 at 13:14
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The reason an impedance mismatch radiates EM waves is because not all of the energy is delivered to the source/load. That energy is converted into EM radiation, and the transmission line works as an antenna.
Antennas have the important property of reciprocity. This means that if you reverse the transmitter and receiver antennas, you get the same result. So yes, if the same EM field is present at the transmission line, it will generate the same current/voltage. To clarify: this is the current/voltage not absorbed by the source/load.
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