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M24SR antenna behaves like a short-circuited transformer when RF disabled

Question asked by mischuk.vadim on Oct 18, 2017
Latest reply on Jan 24, 2018 by JP Miller

Hello everyone. My current project requires working with NFC and RFID interfaces. I use ST M24SR to interact with NFC-powered smartphones, NXP PN532 to read 13.56 MHz cards and inhouse-designed circuitry for reading 125 kHz em-marine cards. So I have three antennas put one inside another. I turn each of these ICs for short moment of time to scan for appropriate tags or phones. If I turn two of them simultaneously collision occurs and weird behavior happen. 

When M24SR antenna is situated aside others everything seems to be OK. But when I put all antennas together as required by device design I observe following behavior: when I disable RF frontend on M24SR using RF disable, pins AC0 and AC1 behave like a short circuit and act as pretty heavy load for other antennas. So, when activated M24SR is able to interact with smartphone, but all other parts of my device cant properly work. I was thinking about putting RF switch in series with AC0 pin. I am not familiar with all that RF tricks like antenna matching and I am not sure on RF switch endurance (in my design it should switch every two seconds and device should work 24/7), so I am asking you guys, if anyone have better ideas.