DC/AISG By pass meaning in diplexer/triplexer/ hybrid combiner

[Saunders]

Hello All,
Could you help to suggest me about meaning of "DC/AISG bypass" that are showed in the triplexer/diplexer/hybrid combiner equipments?
And may i know in which situation we will use it?
9-29-2012 12-24-21 AM.jpg
Thank you very much.

[Stantheman]

It would appear to be DC voltage bypass for powering equipment further up the antenna system after Port 4, say, a tower top amplifier arrangement. As you can see the three filters are AC coupled, they will not pass DC current, so a DC bypass is needed, which presents a high impendance to your RF signals but allows DC current to flow to Port 4. The 3 x DC bypass arrangement in the left hand design in the drawing, would want the 3 DC feeds to all be the same voltage, as they are being combined into port 4, this would allow current sharing between the DC power supplies but more importantly - redunadancy for supply of DC power to port 4. In this arrangement the dc power supplies would need reverse polarity protection, if one was to fail then the others would not drive DC into it.

That is my take on the drawing, others may have good comments.