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I'm working on a phased array radar design, and I'm trying to work out the best way to combine the different receive signals from various antenna. I'm also trying to reduce cost as much as possible, so reducing the number of RF switches, active components etc is important.

The frequency range of interest is 2-3GHz, and the signal will be narrow band (that sweeps 2-3GHz).

Each element is a transmit and receive module (TRM) and can be switched from one to the other by serial control. This means I need to be able to selectively combine any number of RX signals from the total (for the sake of argument 16).

Simply connecting all signals into large combiner or cascade of combiners is going to lead to the situation where some of the input ports are driven and some are floating, connected to a length of copper that is unterminated at the end. Will this cause issues and am I likely to loose power up the floating track of the unused input port? Will the port-port isolation help alleviate this?

Is there a particular type of combiner that is most suitable to this kind of scenario? Something that can be realized by PCB etch and passives would be absolutely ideal.

I can add in a switch to a short terminated stub so that unused input ports are terminated, but that's a lot of extra RF switches that I'd rather avoid if possible.

Thoughts?

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  • \$\begingroup\$ If reducing cost is important then it's quite simple. If there is a monetary benefit for doing this project then maybe spending more on the design and the build will produce bigger rewards. If on the other hand there is no monetary benefit then drop the project. However, if the benefit is learning then this can be monetized so spend more. \$\endgroup\$ – Andy aka Feb 23 '16 at 17:43
  • \$\begingroup\$ I'm not an RF guy, but can't you just put 50 ohm terminators on the unused inputs? \$\endgroup\$ – George Herold Feb 23 '16 at 17:51
  • \$\begingroup\$ @GeorgeHerold, I think he wants to programatically change which inputs are used. But I think there is a way to use your suggestion to do that. \$\endgroup\$ – The Photon Feb 23 '16 at 17:57
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Combiners that have isolation between ports (so hybrids, wilkinsons etc) also have at least 3dB loss per transit. The interport isolation means that you do not have to worry about terminating all ports, the 'in-use' ports will be isolated from the reflecting ports. However, the loss of a cascade of these things will mount up. They also require a fairly large bit of board real estate for printed couplers.

Try looking at RF switches, for instance Perigrine (I don't have shares in them, but used them a lot when designing RF stuff). Large portfolio of very tiny very good RF switches.

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George's comment gave me an idea. Maybe something like this?

schematic

simulate this circuit – Schematic created using CircuitLab

R1 just indicates the output impedance of a matched amplifier. The switch can potentially be a low-cost pin diode.

Maintaining phase stability and not adding excessive noise from the buffer amp might be a killer though, depending on your application.

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  • \$\begingroup\$ This was kind of eluded to in my original post anyway, It's something I was trying to avoid because of the additional cost of the switches (an extra switch per input soon stacks up), but it looks like it might be a necessary evil. I would probably terminate the unused input through 50 ohms however to keep impedance. \$\endgroup\$ – Oliver Feb 24 '16 at 9:21

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