TDR Measurement Using a VNA or Oscilloscope
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The bit rate of digital communication standards have dramatically increased in recent years, with multi-Gbps transmission becoming commonplace. For example, USB 3.0 specifies a bit rate of 5 Gbps. With this increase in bit rate, issues not seen in traditional digital systems arise. Issues such as reflection and loss cause distortion in digital signals leading to bit errors. Timing skew of signal paths becomes critical as the acceptable time margin for proper device operation decreases. Radiated electromagnetic waves and coupling due to stray capacitance cause crosstalk leading to errors in device operation. As circuits become smaller and compact, this issue is magnified. To make matters worse, the trend towards lower supply voltages leads to lower signal-to-noise ratio, and more sensitivity of device operation to noise. Although these issues increase the difficulty in digital circuit design, there is constant pressure to shorten development time.