# GPS splitter with 200Ω DC load and 50Ω RF impedance on all ports

How can the GPS400 signal splitter provide a $200~\Omega$ DC load and $50~\Omega$ RF load to GPS receivers on all four ports?

Most GPS receiver modules require an active antenna. As a result, most receiver modules provide a DC bias on the RF connector to power said active antenna. These modules typically implement an "antenna fault" detector circuit that checks for a DC load. The assumption is that if there is a load, the the active antenna is present.

When working with a simple:

GPS module <-> cable <-> Active antenna

everything works as expected.

The problems come up when there are multiple GPS receivers that all need to use a single active GPS antenna. In my case this would the antenna mounted outside the building with a single cable running in to the lab.

I was able to find the GPS400 splitter to do just what I need. The GPS module that will power the antenna is connected on port 1. Ports 2,3,4 are then presented with a dummy 200ohm load to prevent an "antenna fault" detection on the remaining modules.

What I understand:

• It uses a Wilkinson Splitter to split the RF signal while preserving the 50ohm input impedance.
• The use of DC blocks on ports 2,3,4 to prevent the DC bias from passing through to the input port.
• The use of a $200~\Omega$ resistor to ground which presents a DC load for ports 2,3,4.

The closest to a schematic from the datasheet is this:

What I do not understand:

• Why does this configuration not affect the output impedance on ports 2,3,4?
• If there is a $200~\Omega$ resistor to ground in parallel to the $50~\Omega$ antenna, shouldn't the output impedance be $40~\Omega$ instead?
• The $200~\Omega$ resistor to ground is likely fed from a high impedance transmission line, so it looks like a reasonably big inductance (i.e. RF choke). – Captainj2001 Jul 21 '16 at 21:39
• @Captainj2001 Ahhh.. that makes perfect sense. Why don't you make this comment an answer? – Pablo Maurin Jul 25 '16 at 21:22
• Well, this is only one hypothesis, there are a number of other techniques that could potentially be employed, the one I mentioned previously is just the most likely. There are better biasing implementations using both high impedance lines and radially tapered open stubs on the bias line. – Captainj2001 Jul 25 '16 at 21:34
• I will add a complete answer when I can though! – Captainj2001 Jul 25 '16 at 21:40
• Alright, that settles it. I need to learn a lot more about transmission lines and RF in general. Thanks for the help. At least now I understand what I should be looking for. – Pablo Maurin Jul 25 '16 at 21:43

• You didn't address how the 200 $\Omega$ and 50 $\Omega$ interact – Sean Houlihane Nov 1 '16 at 12:03