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I'm working on a design for an RF switch using latching relays to switch two transceivers between four antennas. Each transceiver will be connected to at most one antenna at a time. But suppose one of the relays fails, and both transceivers are connected to the same antenna. This could result in damage to one of the transceivers.

What is the simplest way to prevent against this condition? Can I use one set of contacts on each relay to verify the state of each relay? I was thinking of doing something like this and applying AND or NAND to the control circuit based on the state of the relay's contacts.

I imagine there is a best practice for doing this but I haven't been able to find any write-ups about it.

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  • \$\begingroup\$ You are aware that you'll need special relays designed for the right impedance at your RF frequencies? \$\endgroup\$ – Asmyldof Sep 7 '16 at 7:09
  • \$\begingroup\$ Yes, I've found suitable devices. Thanks for the tip though. \$\endgroup\$ – Newton Rogers Sep 7 '16 at 17:09
  • \$\begingroup\$ Just checking, you would be surprised how many people would come back 2 weeks later with "I had this question, and built the answer with TE small signal toggle relais and now my antannae don't work at all. Fix it now, plx" \$\endgroup\$ – Asmyldof Sep 7 '16 at 17:35
  • \$\begingroup\$ Hah, yes I probably would have made that mistake if I hadn't stumbled upon RF relays with specific impedances listed. Out of curiosity, would it only make the circuit an unintentional impedance transformer? Or would there be other negative effects as well? \$\endgroup\$ – Newton Rogers Sep 8 '16 at 18:27
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Switching logic.

I have no idea how you would use relays in an antenna circuit but the logic you require is shown in Figure 1. It uses only two changeover relays - one single-pole and one two-pole.

How to design a safe RF switch using latching relays?

You don't explain why you need them to latch.

I was thinking of doing something like this and applying AND or NAND to the control circuit based on the state of the relay's contacts.

If the electronics fails then your circuit fails in an unsafe manner.

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  • \$\begingroup\$ "You don't explain why you need them to latch." So that if power is removed during transmitting, the relays won't change state, potentially causing damage. \$\endgroup\$ – Newton Rogers Sep 7 '16 at 6:21
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But suppose one of the relays fails, and both transceivers are connected to the same antenna.

You design the transmitters to only transmit the power they are supposed to and if something is taking to much (or too little) current then you detect this condition by monitoring the power rails to the transmit power amps.

In other words protect it by making it reliable under fault conditions. The monitoring of current for each transmit PA can tell your control circuit if there's a problem with the relays.

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  • \$\begingroup\$ The transceivers have already been designed. The design goal here is to design a switch that will not cause a cascade of failures if it fails... \$\endgroup\$ – Newton Rogers Sep 7 '16 at 17:10
  • \$\begingroup\$ You can monitor the current into the transceiver and actively limit it to protect it AND use the detection of current limit to warn the control system. You will not find a mechanical relay that is as effective as a non-mech circuit. I'm offering the preferred solution as I see it. \$\endgroup\$ – Andy aka Sep 7 '16 at 17:27
  • \$\begingroup\$ Ahh, OK, do you have a recommendation for how to do the current detection? I suppose I could just detect forward power on each RF input? \$\endgroup\$ – Newton Rogers Sep 8 '16 at 18:26
  • \$\begingroup\$ Just look at the DC current taken on the power rails. I don't know what you mean by "detect forward power on each RF input". \$\endgroup\$ – Andy aka Sep 8 '16 at 23:12

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