susceptible to noise. When bandwidth increases, two transmission parameters are affected: rise time and insertion loss. Rise time is the speed a voltage transitions from one level to another. With faster rising edges, reflections in the channel become stronger and noisier. Simultaneously, higher frequencies have higher insertion loss, and more insertion loss means less signal. In summary, for 800G and 1.6T data rates with OSFP, the modulation lowers the available signal magnitude, there is more noise from higher reflections, and there is less signal from higher insertion loss. Interconnect manufacturers provide solutions to overcome these problems, such as the use of products designed to bypass PCB traces with twinaxial cable. Figure 2 displays typical applications for the cabled transmission line product families.
To show how cable can improve insertion loss, two metrics are presented: the loss reduction multiplier of using cable and the insertion loss of the cable over frequency. At 200G and 400G data rates, transmission line lengths can be about four times as long by using cable, and at the 1.6T data rate the length reaches can be over six times as long. For 200G and 400G, the length of the transmission line could be achieved with PCB alone. So even though there was an advantage to using cable, there wasn’t a need. Now at 800G and 1.6T data rates, the signal loss is so high that the system will not work with only a PCB and, at these speeds, the advantage aligns with the need.
Figure 3: Relative change between cable insertion loss and PCB insertion loss
Figure 2: Cable transmission line applications
Figure 3 and Figure 4 show how these solutions improve insertion loss performance. Insertion loss is reported in decibels (db), and anything above 0 is gain. Since interconnects are passive devices, all insertion loss is less than 0 db. The higher the number for insertion loss, the less signal is available. For this reason, insertion loss is sometimes reported as a positive number to make the name match the chart. However, it is standard practice to report insertion loss as a negative number.
Figure 4: Cable insertion loss and PCB insertion loss
By carefully designing the interconnects, reflections can be mitigated. As an example, the reflection parameters, impedance, and return loss of the
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