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TRF7970A antenna design

Other Parts Discussed in Thread: TRF7970A, TRF7960A

Hi,

I want to implement a currently existing antenna to TRF7970A. Here is the datasheet of the antenna that I will use.

 But the recommended EMC filter circuit in the antenna datasheet is different with the one in the datasheet of TRF7970A (P.15, Figure 4-2. Application Schematic – SPI With SS Mode MCU Interface)

May I keep the EMF filter in the datasheet of TRF7970A but not using the recommended EMC filter circuit in the antenna datasheet?

Also, as there is only one TX pin in TPF7970A only but there are two TX pins in the antenna datasheet. Is it a problem if I connect TX2 to ground? 

Thank you for the help!

  • Hey Shirley,

    Sorry for the delay.  Please still use the EMF/Signal Conditioning on the TRF7970A EVM, and remove the EMC on the antenna.  

    Shirley Fu said:
    Also, as there is only one TX pin in TPF7970A only but there are two TX pins in the antenna datasheet. Is it a problem if I connect TX2 to ground? 

    Yes, connect on side to ground.  

    Our EMC circuit is tuned to 50 Ohm impedance, so the antenna side needs to be as well.  I'm not sure if the pre-populated values will work or not.  It may need to be re-tuned, and you will need a damping resister to bring down the Q.  

    If you are just looking for a off the shelf antenna, Taoglas has several options that are already tuned for 50 Ohms.

    Thanks,

    JD 

  • Thanks John!

    I may ask Abracon whether the antenna is 50ohm or not as I wish to have a small antenna due to the limited of space. If not, Taoglas is a good choice for me.

    But for the Abracon antenna that I want to use, the minimum Q-factor it suggests is 30. Is it a problem? I may need to decrease the BW to increase the Q factor. However, the recommended minimum BW is 2 MHz only.

    Really thanks for your help!

    Shirley

  • also see this post - i did example of matching with Abracon antenna

    Edit: link broke, pasted below.

  • Shirley,

    Since you won't be able to access Josh's link, here is what it linked to:  

    Josh Wyatt said:
    the matching network back to 50 Ohms for this antenna will start by looking at the datasheet for the ANFCA-1510-A02 antenna.
    they list this one as 1.8uH, with a DC resistance 1 Ohm, which equates to a complex impedance of ~ 1+ j153.2822. 

    for a ending Q of about 7, we can then do ballpark calculation for ISO14443A/B and FeliCa operations which result in using a parallel(shunt) resistor across this coil with a value of 1k Ohm. 

    using this resistor value gives you a parallel capacitor value (to start with) of 24pF and a series value of 56pF

    for a ending Q of about 16, we can then do ballpark calculation for ISO15693 operations which result in using a parallel(shunt) resistor across this coil with a value of 2.4k Ohm. 

    using this resistor value gives you a parallel capacitor value (to start with) of 41.5pF and a series value of 36pF

    with regards to the schematic - 
    R243 (on EN2) could be pulled low instead of high
    R247 (pull-up on I/O_4) is most likely not needed
    R248 (pull down on 50Ohm line) not needed
    C135/C136 are the series caps i mention in previous post 
    C229/C137 are the parallel/shunt caps i mention in previous post
    R249 is the parallel(shunt) resistor i mention in previous post

    Thanks,

    JD

  • Thanks Josh and John!

    I understand now!

    But I want to confirm that it is not a problem if the bandwidth is lower than 2MHz?

    As Q = F/BW =R/XL, when Q increase, BW will decrease.

  • Shirley -
    Correct -
    Bandwidth (BW) for ISO14443 based operations should be set around (as close to) 1.68MHz to result in a Q of about 8
    Bandwidth (BW) for ISO15693 only based operations should be set around (as close to) 848kHz to result in a Q of about 16.

    if using multiple protocols (i.e. polling for ISO14443A/B and ISO15693), go with the lower Q value as target for BW. ISO15693 will still work fine.
  • Yes,I am also working with same TRF7960A controller for my NFC application using external ABRACON antenna.

    And I want to build same matching circuit.

    So, Can you update the Schematic no of resistor and capacitor which respect to this updated schematic.
    link for schematic : www.ti.com/.../trf7970a.pdf
    Page No:76,Figure 7-2:Application Schematic :SPI With SS Mode MCU Interface