Why is 38 MHz typically the highest frequency used for upstream modem transmissions?

The choice indicating that 38 MHz is usually the highest frequency for upstream modem transmissions is generally linked to the limitations of diplex filters, which can introduce group delay. As signal frequencies increase, the ability of diplex filters to handle these frequencies without distortion becomes constrained. At 38 MHz, the upstream signals are pushing against the operational limits of these filters, causing potential delays in signal transmission that can affect communication quality.

In the context of cable bandwidth, the upper limit determined by the physical properties of the medium and the technologies employed does not typically extend beyond 38 MHz for upstream signals, making it a significant threshold related to transmission capabilities, not just filter behavior alone. Although factors such as signal clarity and compatibility with standards are important, they do not directly tie into the technical limitations imposed by the filters that manage frequency separation in modem transmissions. Thus, understanding the effect of group delay from diplex filters provides insight into why this frequency is often chosen for upstream modem transmissions.