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92 DESIGN007 MAGAZINE I AUGUST 2020 tion not being optimized. If the testing system knows how much energy is sent to the circuit and how much energy exits the circuit, it will assume the losses (insertion loss) are due to the circuit only. But if the return loss is poor, then much of the insertion loss is not due to the loss of the circuit but is actually due to the reflected energy that was never resident on the circuit. The insertion loss measurements of the circuit are not accurate with poor return loss. For time-domain aspects of HSD, imped- ance transitions may or may not be an issue, depending on the digital rate, rise time, and the sensitivity of the circuit. For a circuit oper- ating with relatively slow rise time, impedance transitions have much less impact on the per- formance of the digital circuit. However, when the rise time is much faster, the circuit is more sensitive to small anomalies often associated with impedance transitions; then, the HSD performance of the circuit could be negatively impacted. The digital rate (speed) and rise time are related to analog or RF wave properties. The simple square-wave of an HSD clock signal is generated by summing an RF wave and its upper harmonic waves to create the square wave. That means for a slow digital speed, the RF waves used are relatively low frequency. For example, a digital rate of 1 Gbps has a fun- damental frequency of 0.5 GHz, and the next several harmonic waves will be at 1.5 GHz, 2.5 GHz, and 3.5 GHz. At those frequencies, return loss is typically insignificant for most PCB applications. Additionally, the impedance anomalies associated with impedance transi- tions for frequencies and digital rates that low, are typically not a concern. However, for a vHSD application running at 28 Gbps, it will use waves at 14 GHz, 42 GHz, and 70 GHz. At 42 GHz, return loss and the associated impedance transitions are very important, and at 70 GHz, these concerns are critical for mmWave applications. These RF issues can have an impact on the eye-diagrams for vHSD, but from limited experimentation, the effects have not been as dramatic as I would have expected. However, for a sensitive vHSD system running at this speed, the return loss and impedance transitions should be con- sidered. The effect of return loss and imped- ance transition may make a difference for eye- diagram performance in higher-speed vHSD circuits, which operate at 56 Gbps. Understanding mmWave issues in more detail is highly recommended for the designers of vHSD applications. There are several places where more information can be found for mmWave technology online, including many resources to learn more about the practical aspects of mmWave technology. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coonrod, click here.

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