Technetters, I would like to know if there exists some differential impedance standard. I have an airline, which presumably works well in checking the individual channels of a TDR.

Also, I have saved a differential test coupon, from internal production, which gets measured every 2 weeks on the TDR and the data recorded in an SPC table. This approach aims at detecting any changes in the measurement process, e. g., damage to the TDR. The question has come up more than once, 'What is the true impedance of that differential coupon?' Incidentally, the airline gets measured at the same time, by the individual channels of the TDR.

Tangentially related to the above, the SPC chart for the differential coupon suggests the impedance has dropped 0.2-0.3 ohms since February 2012, or something has changed in the TDR. The SPC charts for the airline and the two channels do not indicate any change in them. I am wondering if increase in humidity as the year passed could have led to an impedance decrease. The coupon material is FR4.

Thanks for any guidance.

Louis Hart
Compunetics
Monroeville, PA
USA
[log in to unmask]<mailto:[log in to unmask]>
412-858-1232


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