Can anyone help with where and how I should inject the modulator signal, looking at the diagram below, without killing the cable signal or the modem?

Thanks,

JJP

I'm assuming that this is not a digital cable system, 'cause if it is you're SOL. You should be able to shunt one of the 3-way outs, use one for the cable modem, then put the other into a 2-way combiner (along with the modulated signal). Then take the combined output, put it into another 2-way (spitter this time), then into the two RF amps.

Another option would be to just split the modulator output and take individual feeds into the two RF amps (provided your diagram is correct and the RFA-2000 has a second input).

If the RF amps can be cascaded (that is if their inputs can handle the signal), I would use one output from the broadband splitter into one amp, then out into a combiner with the modulator, into the second amp, and the split four ways.
You may need a low pass filter before the first amp, depending on how clean your cable providers' signal is, and don't forget to terminate the unused output on the broadband splitter, or swap it out for a two way.

How about splitting the incoming signal 2-ways, one to the modem, the other to a combiner. Combine incoming cable, modulator 1, modulator 2, etc. However many inputs you may need...then split that signal to the two D/A's?

These are all correct.

As for digital cable, you are not quite SOL if the client does not actually want to WATCH digital cable, but just has it on the cable. You can use a Channel Plus low-pass filter that passes channels up to 86, then kills signals above that, which includes the digital cable signals. Modulate from 90 on up to 125, skipping the upper 90's channels in the FM band.

Or, if some TVs are to get digital cable and some are to get modulated signal, mix the modulated signal in before an amp but after the bottom splitter, and use the low-pass filter only on that amp.

You will want to experiment, though, with the modulator's exact injection position. Some modulators have such high output that they should be mixed in after an amp (as in that last paragraph) or they need to be attenuated so as not to overload the input of the amp(s). Overload shows up as herringbone on some channels, some channels may even look snowy....