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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