Does a mux IC (16x1) affect the impedance of an antenna ? I was thinking of interfacing 16 antennas to a mux and read just one of them through the S6350 reader. My first solution was to use a MUX (16X1), but my professor said that wont work becuase there is high chances that the impedance missmatch thus making the antenna useless. The antenna RFID reader and the tag operates at 13.56 Mhz. My question to you is how I can make 16 inputs to one output( attached to the reader)? Is the things he is saying true ? I have seen other projects using muxes with antennas but he was saying it was a lot of active componenets inside a mux that can change the impedance thus make the antenna useless.
Traditionally RF switching is done by a PIN diode switch like this one:
(Picture from this site)
Depending on wheteher the Diode is forward/backward biased RF passes through or not. You have to take care that biasing voltage is larger than RF peak voltage.
Several such switches can be used to multiplex several antennas to one transmitter/receiver.
E.g. like shown here.
There are also integerated circuits containing several diodes for such purposes.
Many factors go into selection of an antenna switch, which you must determine based on the antenna impedance and calibrated load capacitance if used , signal level and frequency.
From this your MUX switch must provide >> 14MHz , ON resistance much lower than antenna impedance (<2%) , minimum ON and OFF capacitance so as not to disturb antenna resonance and ,inputs minimum charge injection during switching (pC), so as not to disturb Rx AGC with a large transient pulse.
Unfortunately your requirements are unknown and these charges often imply design tradeoffs e.g. Ron vs C pF vs cost.
Here is one potential candidate after you define the antenna specs for all signal levels, impedance , load pF.
Also the DC range of voltage must be within the acceptable supply range for these switches for both TX and Rx
It is not impossible but hard to do so and not economical. Here is an outline of what it takes:
First off, you can't simply switch the antenna. There is a impedance match network between the antenna and the NFC chip. The switch itself will detune the network. The different length wires to the antenna will detune the network as well. It is unlikely that you can just put a mux before the antenna and the reader will still work. The matching network has to be re-done.
Furthermore, the network will do an impedance transform which turns the low-voltage chip TX signal into a voltage that drives the antenna resonance tank. At this point you can easily have 50V or more voltage. Finding a mux for that will be hard.
It is much more practical to switch the transmit (TX) signal right after the NFC controller and use a match network for each individual antenna. Doing so is a lot of work.
Furthermore you have to switch the receiver signals. They carry a weak and very noise sensitive signal so long wires are a no-no. You need buffer amplifiers for them close to the match networks to minimize the signal loss. Use shielded cables to minimize cross-talk from the TX signal.
Technically all of the above is doable, but I bet it's cheaper and more practical to just build a tiny reader for each antenna and be done with it within a few weeks rather then spending a year getting all the matching networks working.