Multiplexing NFC antenna on CR95HF

[craig_orv]

Hello,

I am using a CR95HF NFC transceiver and want to have the option to select one of two antennas in my design.

I had protoyped this idea with some Skyworks RF mulitplexers and a CR95HF development kit, essentially tapping into where the antenna joins to feed it through the mux. I'm using two off-the-shelf NFC antennas at the other side of the muxes.

Unfortunately this hasn't worked. I'm not sure if I'm doing something wrong in my setup, or if the two new antennas simply are not tuned correctly to work with the existing dev kit.

Thoughts and insight appreciated!

[smithderek2000]

I'm multiplexing one ST25R3916b IC to 15+ antennas. Currently I have it working great with a small signal DPDT relay. I recommend using that approach. However to save cost I'm also playing around with a TI TMUX6221 analog switch IC. I'm testing right now with the ST25R3911 Discovery board since it's set up for this. With the antenna going through it, mine will read a RFID tag but won't calibrate the antenna successfully. I reduced the output resistors from 4 ohms down to 1 ohm to get higher signal, too.

[craig_orv]

I know a relay would probably work, but I want to avoid a click-clack in the product which is otherwise entirely solid state.

We could really both do with some input from STM on this.

[smithderek2000]

Ha - that's exactly why I'm replacing the relays now. They worked great even with lots of channels, but the customer didn't like the sound. So now I'm using the TI analog switch. 

I've asked multiple times but have not received any help from ST. I think that Antenna Multiplexing would make a great app note.

[Henry Crane · Accepted solution]

Hello,

RF switches are designed to operate with 50ohm loads. Even if they are equivalent to low impedance pass transistor, there can be some frequencies where non pure real load impedance can be transformed into different impedances because of resonances. S parameters of the switch don't seem to be available so anticipating the effective input impedance of the switch when loaded directly with an inductive load is not possible. Unless the differential impedance between NFC+ and NFC- is equal to the impedance our demo board antenna (0.50hm in series with 1.1µH), the CR95HF antenna tuning circuit has to be modified to match with you setup. so in a first step, I recommend you to assess the NFC+ - NFC- impedance and then modify the CR95HF antenna tuning circuit.

Unless you really need to have only one antenna active at a time, another solution is to use the TX1/RX1 pair and TX2/RX2 pair as two single ended input/outputs. L4/C9/C10/C8/C1 can be the tuning circuit for antenna 1 while L5/C13/C14/C12/C15 can be the tuning circuit for antenna 2. of course, Ca differential capacitor has to be removed and replaced by a dedicated parallel capacitance in parallel of each antenna (between signal and ground). If you still need to activate only one antenna at a time, you can keep the RF switches connected them as as follows:

- C2a is removed

- add a NFC+ to GND cap to replace C2a for antenna 1

- add a NFC- to GND capacitor to replace C2a for antenna 2

- C1/C2/C3/C4/C5/C6 can be eventually replaced by 0 Ohm resistors  (C8/C11 and C12/C15 act as DC block for RFC inputs, and antenna does not generate any DC)

- connect antennas between RF1s and GND of each switch (of course NFC-GND inpout impedance of each switch has to be known to adapt matching circuits.

By doing so, switches input and outputs are working in proper single ended mode which is better than using the switch in differential mode. the draw back is the RF available power that is divided by two.

feel free to ask for more details if necessary.

best regards,

Henry Crane RFID/NFC product technical support.