Some systems routinely install an eight-way split at the output of each return optical receiver, terminating the unused ports. That way, when they add a return service, they neither interrupt existing services nor change levels when they connect the new system's receiver. Return systems, including headend RF management, must be designed and balanced, just as you expect to do for your downstream plant. If any operating levels are changed later, you have to be aware of the effect on the overall system. We know that keeping all interference out of the return path is difficult, but we also know that it is possible to live with the interference if we understand it and take the right steps to minimize its influence.
Most people report that interference comes mainly from homes, and secondarily from drops.
Little interference originates in properly maintained hard line plant. Thus, the issue often becomes one of applying the right countermeasures to interference coming from the home and maybe the drop. Figure 2 illustrates options that have been used or proposed as countermeasures against interference originating in the home or drop. Option 1 is a high pass filter installed at the tap or the home. The filter removes interference in all or part of the return band.