For the past several years, I have been working with many different millimeter-wave (mmWave) applications. I have also been working more with high-speed digital (HSD) applications, and when looking at some of the data, I had become somewhat frustrated. Sometimes, the insertion loss curves generated for HSD applications have a tremendous amount of noise, and the few times I investigated, I found the testing results had poor return loss, and the HSD engineer was not concerned. From my mmWave background, that really disturbed me because return loss is one of the critical properties for obtaining valid data. However, as I have learned more about HSD, I find the technology is commonly focused on the time domain—and return loss has much less influence on most time-domain performance issues. With my learning curve improving for HSD—and more specifically, very high-speed digital (vHSD)—I now see a lot of areas where understanding the tricks of mmWave technology could help improve design and performance for vHSD technology.
Impedance transitions for mmWave are extremely important to characterize, and this is typically done to get the best return loss behavior for the circuit. Just as a quick side note and clarification, return loss is also sometimes called reflected loss and refers to how much energy is reflected from a propagation medium. For example, the transition from the connector to the circuit usually has an impedance anomaly. If that is not well characterized, much of the energy that would be desired to be inserted on the circuit will get reflected back to the source due to poor return loss (reflected loss). In this case, the poor return loss is probably due to the impedance transition between the connector and the circuit, and that transition 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, impedance transitions may or may not be an issue, depending on the digital rate, rise time, and the sensitivity of the circuit. For a circuit operating with relatively slow rise time, impedance transitions have much less impact on the performance 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 fundamental 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 transitions 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 considered. The effect of return loss and impedance 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.
This column originally appeared in the August 2020 issue of Design007 Magazine.