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Six tips for better TDR measurements

Signal speeds and devices continue to get faster and operating voltages continue to fall, placing ever increasing demands on signal and power integrity. If you haven’t already been measuring PCB traces and planes, you will certainly have to in the future. Fortunately, time domain reflectometry (TDR) instruments are very simple and fast to use. By following a few simple guidelines, your measurements will be high fidelity and easy to read. #1: Use high quality, low loss cables and connectors Inexpensive cables are often acceptable for making basic measurements, and we all like to save a few bucks. But this is not the best place to cut corners. You should only use only high-quality, low-loss cables or special TDR probes if you want to achieve accurate TDR measurements.

The setup in Figure 2 illustrates the impact of poor quality connectors.  In this image, the TDR is deskewed with no cables attached to set the reference plane at the TDR instrument front panel. A high-quality semi-rigid coax is connected to the front panel while a thru connector is used to attach a second high-quality cable.  Finally, the second cable is attached to a PCB 50Ohm test trace with a low-quality PCB edge connector.

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