In a former life, we measured coupons for spc/reference daily for 28, 50, 100 ohm single ended, and 100 ohm differential.  Two weeks is a bit of a stretch in my opinion, because the scopes change daily based on temp/RH.  Not to mention, the cabling, static isolation components and probes all wear out. The contact points on the coupon also wear out. Add to that different operators have different techniques and end up with slightly different measurements.

Craig Sullivan
Manufacturing Engineer
MPL Incorporated
P: 607.266.0480 x115
F: 607.266.0482
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Louis Hart
Sent: Tuesday, September 25, 2012 6:46 PM
To: [log in to unmask]
Subject: [TN] Differential impedance standard

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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