What is the best way to terminate long ST25R3916B antenna traces?

[smithderek2000]

Hello, 

   I have one ST25R3916B that feeds a few antennas spread out over 23 inches of length, using relays so that only one antenna is connected at a time. The ST25R3916B is located in the middle of the length. Traces are 20mil wide with ground plane cleared out underneath. I'm getting good results on one half of the board but the same antennas on the other half are not reading as well. I'm thinking it's reflections on this line.

So what would be the recommended way to terminate the 13.56MHz differential antenna signals on either end?

Thanks,

Derek

[Travis Palmer]

Hello Derek,

 

Maybe terminating alone is not giving the best results. 

My guess - without seeing the system -  would be to de-tune the un-used antennas to a different resonance frequency. This could be done by connecting parallel capacitors to the antenna, the same way as disconnecting them.

You could also try to shorten the parallel capacitors. 

First i would recommend to look at the difference of the RFI signal between good and bad antenna. Is there any obvious difference (carrier strength, modulation/wave shape, LMA from the tag)?

 

BR Travis 

[smithderek2000]

Hello,

   In looking at the waveform, how can I tell which is from the tag vs. which is just the excitement signal from the reader?

   Also, I read through AN5276, but how can I verify that my antenna is correct?

The system was working well with one '3916B and 16 antennas, one at a time selected with relays. The client didn't like the clicky-clacky sound of the relays so I replaced them with analog switches, TI TMUX6221. System works fine as a bare board but not in its metal mounting bracket, whereas the relay solution worked fine in the metal mounting bracket. Now I'm trying to figure out why.

Thanks,

Derek

[Travis Palmer]

Hello Derek,

Question #1:

Not sure if I understand your first question correctly. The excitement signal coming from the NFC reader is a square voltage. It is outputted via the RFO1/2. The EMI filter turns the the rectangular voltage into a sinus voltage.

Question #2:  

The typical way to check if the antenna is correct is the smith chart. This is mentioned in the verification chapter in 7.4. Measurement of the target matching impedance.

You can either have the reader un-powered or powered with the driver set to high-Z (otherwise a DC voltage will be present on the RFO`s which might harm or disturb your VNA. 

The switches should be at their final state (correct antenna selected).

Here it will be interesting to see the difference between relays and RF switches as well as with and without metal housing. 

From your explanation it seems strange that the housing has an influence on the RF switches and not on the relays.

 

please feel free to share the pictures of the smith chart.

 

BR Travis