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How do I do differential TDR measurements?

Question :

How do I do differential TDR measurements?

Answer :

First of all, you need to make sure that your signals arrive to the DUT at the same time. You can follow the procedure in the oscilloscope manual and having observed the difference in delays between the two channels in differential mode, adjust for half of that difference by using delta TDR function in the scope. However, we prefer to use an alternative procedure, whenever possible. To follow this procedure, you should use an absolutely symmetric device; our preferred reference device is a 50-Ohm power splitter/combiner; it is short and symmetric. Such device should not generate any common mode response from differential stimulus when the differential signals arrive at the two terminals of the combiner. Shorting the two probes or SMA connectors together for this procedure is acceptable as well. Thus, connect the scope in differential mode to the reference DUT, and add Channel 1 and Channel 2 on the scope, which will allow you to observe the common mode response. Use the delta TDR delay function in the scope to adjust the relative position of the TDR sources until your common mode response curve becomes completely flat. Your cables/probes should be reasonably matched in length, but the delta TDR delay allows you to adjust the relative position of the TDR sources by about 400ps and correct for small difference in probe/cable length.

Then, you only need one reference waveform, acquired with the scope in differential mode; both probes are connected together to ground (or, in case of SMA connectors, the two TDR lines are connected just together using an SMA female-female "barrel" interconnect). Acquire channel 1 on the scope only; no scope math is required.

Then, connect to the DUT, keeping the scope in differential mode, and acquire channel 1, no scope math is required. This is your odd waveform. Switch the scope into common mode. Acquire channel 1, no scope math is required. This is your even waveform.

Proceed to compute even/odd impedance profiles if you wish to get the differential transmission line model as described by the IConnect software manual.

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FAQ ID 56551

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