TDR Measurement Considerations

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Channel and Trace Concepts

It is important for oscilloscope users to note the difference in the concept of channel in oscilloscope and network analyzer.

In network analyzer, channel is referred to the window for displaying traces. Because a channel corresponds to a window, it is called a channel window.

On the other hand, the points on the front panel of the network analyzer where cables and DUTs are connected are called test ports. These test ports are not channels and do not directly associate with channel windows.

The VNA, for example allows you to use up to 150 channels to perform measurements under 150 different stimulus conditions. All traces that are assigned to a channel share the same channel settings.

To learn more about setting the channel and traces, refer to Traces, Channels, Windows, and Sheets.

Enhanced Time Domain Analysis option allows you to use channel 1 only. Up to 16 traces can be displayed in this option (when Differential 2-Port DUT topology and all T or all S traces are selected).

Device Under Test (DUT) Considerations

Maximum DUT Lengths

To convert from DUT length in seconds to distance in free space, multiply the value in time by c, the speed of light in free space. To calculate the actual physical length, multiply this value in free space by Vf, the relative velocity of propagation in the transmission medium. (Most cables have a relative velocity of 0.66 for a polyethylene dielectric or 0.7 for a PTFE dielectric.)

AC coupled (DC cut) DUTs

Caution is required when measuring AC coupled (DC cut) DUTs.

The response after the capacitor cannot be measured correctly, because the capacitor response overlaps the response from the input and may cause measurement errors.

When measuring AC coupled devices, make sure that the following two conditions are satisfied for proper measurements.

When the above conditions are not satisfied, either:

Test Cable and Connector Considerations

Using high quality cables to connect the DUT is recommended in order to minimize measurement degradation. The cables should have low loss, low reflections, and minimum performance variation when flexed.

Note: When performing deskew and loss compensation, the same type of cable must be used for all test ports, but they do not necessarily have to be of the same length.

When performing deskew or deskew and loss compensation, use a single connector interface type, such as SMA, or Type-N, for all of the ports to obtain the best results.