Hello,
First of all, I must say that I am a beginner in electronics. The project I am working on is to build a system based on an ESP32-WROOM-32E-N16 that can read 16 RFID 13.56MHz external antennas. I chose the TRF7970ARHBR for the RFID Transceiver IC part. The schematic is shown below.

My questions are the following:
- While searching for a transceiver IC, i came upon multiple options : TRF7970A, PN5180 (from NXP) or ST25R95. The last two do use 2 TX pins (and 2 RF pins). What i mean is in the below diagram taken from a manufacturer guide, they are called TX1 and TX2, and this result in some sort of symetrical-looking circuit to feed the RFID antenna.

I saw on the internet a existing board from Eccel called "pepper C1 mux" that uses a mux/demux ADG1607 to switch through 8 antennas. This mux is a differential channels (8:1) x 2 :

They use external antennas and a cable, connected with 3-pin connector JST PHR-3 to the board. GND pin immediatly is connected to the GND of the PCB, and TX1/TX2 is brought to the ADG1607, and that seems to work for them.

In my case, the TRF7970A only has one "TX_OUT" pin to power the antenna. So i basically reproduced a schematic from the manufacturer (Texas Instrument) "TRF7960A RFID Multiplexer Example System" (SLOA167). See the part circled in red on the manufacturer schematic example below :

So what is did is take this schematic, and put a equivalent mux to the ADG1607 (in this case, the ADG706 because i dont need differential channels, because i only have one track) between the antenna's cable connector and the equivalent of C12 on the schematic above. And put 16 molex coax 73412-0114 connectors for my 16 antennas.
This leads to the following schematic, i.e. the one of my project (see the "EMC filter, MUX to Antenna's" part in particular) :


I know it seems stupid but since i have only one TX_out instead of 2 TX1/TX2 pins, i was a bit lost and i would like to have your advices on what i did.
- Maybe the introduction of the ADG706 in the circuit changes the balance of the circuit. What are the informations from the datasheet that i should look for, to account for it? Below you'll find the main properties of this MUX :

- Do i need to make balanced 50 Ohms PCB tracks between the TRF7970A and the molex coax connectors ?
- I'm not sure if i grasped the whole concept of matching circuit for antennas etc, but my understanding is that, if i buy 50 Ohms RFID antenna's (package is antennas and its coaxial cable), i'm immediatly fine and can connect any such antenna to my board and not worry about matching, is that correct ?
- The ADG706 seems to be quite limited in Vmax. My whole board runs with 3.3V, so i guess it's fine ? Or the voltage in the RF part can get higher and i might burn the multiplexer ?
- Adittionnaly, (but that is not really a RF-related question but if someone can answer it, that would be great), i saw the TRF7970A has a "SYS_CLK" pin. In the datasheet, they say :

I don't understand whats the purpose of this. The MCU module has its own crystal and runs at 40MHz. Should i connect SYS_CLK to the microcontroller ? As you can see on the schematic, i'm a bit lost with what to do with this pin.

Any help will be deeply appreciated. Again, please forgive my incompetence.

 

Welcome to AAC.

The transmission line between the transceiver and antenna must be impedance matched. In this case it means the PCB traces, any wires and/or connectors, and the antenna itself must have a characteristic impedance of 50Ω at the frequency of operation.

There are quite a few application notes I have seen concerning PCB layout to achieve the match from various manufacturers. It’s not terribly hard but it does require care and attention to several details. One alternative is to place an appropriate connector as close as possible to the antenna connection of the transceiver and make any runs to other devices or the antenna via appropriate miniature coaxial cable.

You should also keep in mind the high losses in such cable at higher frequencies and select accordingly.

I know this is very general, but I don’t have the time to be more specific and didn’t want to see your question go unanswered. I hope some others will add more. Good luck, and it’s good to have you with us.

 

General cases, if not the mother`s life and death is dependant of that, the antenna circuits must not be MUXed. Take a corresponding tablet one to each antenna and make MUXing softwarically, for that You have plethora of analog, digital or universal I/O at for example Arduino-Mega. But never make muxing at antenna cable.

 

Also, I don't think the ADG706 switch is the proper switch for your application. You need to use an RF switch.

 

what is the voltage at the antenna circuit? i remember measuring some 85Vpp or so on a 125kHz RFID. as far as i know, all RFIDs use series resonance so voltage can be significant.

 

Thank you very much for your answers.

The transmission line between the transceiver and antenna must be impedance matched. In this case it means the PCB traces, any wires and/or connectors, and the antenna itself must have a characteristic impedance of 50Ω at the frequency of operation.

Ok, this is what i was thinking, so I will implement that. I'm designing it with kicad and will order it from jlcpcb, i read i need to take a 4 layers pcb if i want to do some impedance-controlled tracks, it increases the cost but so is it. I will need one of the antenna to be on the main board, so i will have to design it and do the impedance matching process for it. I read i need a vector network analyser to measure that, no idea how it works, it's gonna be another challenge.

General cases, if not the mother`s life and death is dependant of that, the antenna circuits must not be MUXed. Take a corresponding tablet one to each antenna and make MUXing softwarically, for that You have plethora of analog, digital or universal I/O at for example Arduino-Mega. But never make muxing at antenna cable.

I get that. What surprises me is that the pepper C1 mux testboard does use that idea. Also, i found a commercial equivalent system to what i'm trying to build, that also uses multiplexers for the signals from all antennas. If common sense is to avoid that kind of solution, it is strange to me that other people made design choice to do it. I'm probably missing something.

Also, I don't think the ADG706 switch is the proper switch for your application. You need to use an RF switch.

Yep, i saw that RF switches are a thing but i couldn't find a 16:1 equivalent. Switches i found were all 2:1 and since i need 16 antennas... Well i could use a lot of switches but when i saw that using normal analog muxes is possible and is integrated in working solutions, i pretty much went for that. I saw the following graph in the datasheet of the mux i plan to use :

I'm not sure if i understand what it means but it seems that at 13.56 MHz, that component isn't very transparent. That would support what you're saying. What i don't get is that the Pepper C1 mux uses a very similar mux with the same kind of graph.

Anyway, i will look into that idea of having a transceiver attached to each antenna and mutiplex the SPI communication between the ESP32 and the 16 transceivers. The ESP32 should have enough CS pins for the SPI slave selection. I was a bit worried initially because i though SPI was more for on-boards communication but my antennas aren't that far from the main board, around 1.7 meters max, so maybe with a 150mm ribbon cable... It feels like i'm trading one problem for another.

Anyone can answer to the main question of my original post, that was about the single track vs differential tracks to transport the signal from the antenna ? It's probably very basic and would definitely help me.

Anyway, again thx for taking your time to read this, i will definitetly update this thread with updates on the project

 

1) Voltage in antenna: V=Z*i and i=V/Z thus the V=sqrt(Z*N), where antenna Z is well known value.
2) RE:"Anyone can answer to the main question of my original post, that was about the single track vs differential tracks to transport the signal from the antenna" Had siimilar question while my phone operator demolished the nearest tower thus I had internet in the limit of -130 dBm instead of normal -110 dB thus the "corrupted baits" index jumped to 98%. Found one logoYagi antenna from 600-5200 MHz at least for one MIMO input and indeed the mistake count became better, about 15%. Before it to watch the movie made hanging up computer each lovely minute the twice (demanding restarting), after that 5 minutes was OK but at lost stream at least computer not crushed, only freezed (not demanding restarting). Then I found another logoYagi yet bit larger, and that I put into other MIMO slot. With that the problem was solved completely. But after the month the new tower was erected so my internet is working antenna-less again.