When there is no group delay in a system, network or component, all frequencies within a defined given will take the same amount of time to traverse that system, network or component. When group delay does exist, some frequencies will arrive at slightly different times than other frequencies. Most of the time it happens when the frequency response isn t flat. Make sure your upstream frequency response is nice and flat. As well, keep digitally modulated carriers away from band edges.
Diplex filters have a nasty habit of having lots of group delay near band edges.
It doesn t take much in terms of so-so frequency response to reach 200 ns of group delay. I ve seen field measurements that corroborate this. Well, as usual, I ve run out of space before I run out of things to say. Tell you what, I ll continue this discussion next month. I ve got a few more upstream RF problems that will no doubt keep you on the edge of your seat.