Combining/splitting three different RF signals

Question

Not being so fluent in RF, I decided to ask here if my current approach seems doable or how one would try to achieve something like this:

I'm doing research and deciding on an approach for my proto-board, to understand if and how combining signals from three different antennas into single coax and then splitting(filtering) them at the receiving end for different radio modules would even work (and for transmitting as well, so bidirectional).

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More precisely - let's say I have antennas for 1) GPS, 2) 2G/3G and 3) Bluetooth.

I think the frequency bands for them are rather separated, based on the antennas I have:

• Cellular: 800 - 960MHz / 1700 - 2170MHz
• GPS: 1560 - 1590 MHz (1575MHz center frequency).
• Bluetooth: 2400 - 2500MHz

so to me it seems it would be possible to join these into single coax, without greatly disturbing the signals (they are not in the same bands thus interference should be minimal - especially with filtering).

This where my discussion on different approaches begin, so I currently plan to test two things:

a) Bluntly join antenna traces together into single coax after some impedance transforming and filtering.

b) Use a passive combiner/splitter to join the antenna traces together - in hopes for better isolation when signal is transmitted the other way back.

For the receiving end I would mirror these blocks, before any ANT input on the chips.

^{simulate this circuit – Schematic created using CircuitLab}

SO basically:

• 1) Does just joining antenna lines like that make sense or there's smth completely wrong with this idea? (based on the proposed protocols)
• 2) Would using combiner/splitter make this approach more feasible ?
• 3) I'm completely off and I should try smth else

In any way I believe I need filtering for each of those bands for sure - in order to get better SNR on my receiving end.

• GSM filtering is probably more complicated since there's two bands to deal with. That's why it's red on the schematic.

Answer 1

You should use a four-port multiplexer, more commonly known as a triplexer:

As shown in the diagram, a triplexer is a bidirectional device comprising three filters that separate (or combine) the signals from the input (or output) port into band-limited signals at the three output (or input) ports. A properly designed triplexer will present 50$\Omega$ at all four ports across the unit's usable frequency range.

An alternative to the lowpass - bandpass - highpass architecture is to cascade two diplexers, each of which comprises a lowpass and a highpass filter, to produce the desired signal separation.

The specifics of your solution will depend on the required power handling capability and implementation technology (e.g., lumped elements, microstrip, cavity, etc.) An internet search turns up commercial sources for these parts. If you wish to design your own microwave triplexer, you can download Microwave Filters, Impedance-Matching Networks, and Coupling Structures by Matthaei, Young and Jones (part 1 and part 2) from the Microwaves101 web site.