Bob et al, the differential coupon's nominal impedance is 100 ohms. Neglected to put that it the email earlier. (It was getting late, dark, and wet and I needed to get out of here on my bike.) I've been measuring 99.7 last winter and in the range of 99.5-99.4. Those are the numbers from which I got the 0.2 - 0.3 ohm drop. I picked that coupon to hold as a pseudo-standard because it was measuring close to 100, without knowing what the true value was. Louis

-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]] 
Sent: Tuesday, September 25, 2012 7:01 PM
To: 'TechNet E-Mail Forum'; Louis Hart
Subject: RE: [TN] Differential impedance standard

Hi,

 Is my math wrong but .25 ohms out of 50 ohms is like -46db!  Holy cow, what is the spec of the machine?

Bob K.

-----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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