In your software+ there are two techniques for computing lossy transmission line parameters - based on open-circuit reflection and on matched reflection and transmission. Which technique is better?
Open-circuit reflection waveforms do not require the ability to fixture one side of the transmission line. Only a clean signal launch at one end of the device is required. Matched transmission and reflection measurements potentially allow more accurate extraction, provided that care is taken to get accurate measurements, since the loss is very well represented in the transmission waveform. However, taking transmission measurements requires access to the far end of the DUT. It also requires that either the cables being used on the two measurement channels are the same length, or that the two scope channels are skewed so as to account for any delay difference between the two cables. Your oscilloscope manual will provide details on how to de-skew scope channels - the process will be similar to the process used when de-skewing the channels for differential measurement. If cable delay mismatch is not taken into account, the measured delay of the device will not be accurate.
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FAQ ID 56721
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