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Quality factor measurement in custom antenna

Jose Maria Fernandez
Prodigy 50 points
Other Parts Discussed in Thread: RF37S114

Hi all!

I'm relatively new in radio frequency, so I was wondering if someone could help me. I've designed a small 6 cm x 6 cm (2 turns) RFID antenna to be used together with the TRF7960aevm. The antenna is matched to 50 Ohms using L-topology, with a theoretical quality factor of 18. In order to check the quality factor, I measured the return loss of the antenna with network analyzer:

My problem is how to define the bandwidth of an antenna to compute Q = fr/bw. According to the SLOA135A application note "A rule of thumb is that the antenna 3-dB BW is twice the 10-dB return loss bandwidth". Looking at the markers of the image, this would give Q = 14.82, which is strange because prior to putting the matching circuit, I measured the antenna inductance and resistance at low frequencies to set a proper resistor for Q = 18.

Am I doing the bandwidth measurement properly to calculate Q? Can someone clarify where does the rule of thumb of the application note come from?


Cheers,

Jose

  • 0
    TI__Guru*** 135355 points
    Hi Jose,

    It doesn't look like your image attached correctly.

    In any event, yes we use the +/- 3dB points to determine the bandwidth. It sounds like your calculations are correct to me. That you get a Q of 15 using theoretical values for 18 isn't that bad I think - you just need to tweak and adjust the values a bit. We typically have to do the same when tuning antennas.

    Are you planning to use this antenna for ISO15693 standard tags only? That is a pretty high Q factor and will only provide good performance for ISO15693 tags, so just want to be sure you are aware of that. For ISO14443 tags we recommend a Q = 8-10 or so.
  • 0 in reply to Ralph Jacobi
    Prodigy 50 points

    Hi Ralph,

    Sorry, here is the picture I was mentioning:

    Indeed, I'm planning to read  ISO-15693 tags. As far as I've read, quality factors up to 20 are recommended in these cases. It's good to know that these deviations of the quality factor are usual, so I guess that the measure procedure is correct.  

    My goal is to increase the reading range of tiny tags, such as RF37S114, to 3 cm using the trf7960aevm. I have a couple of questions:

    - Selecting a data rate of 1.65 kbps instead of 26.48 kbps improves the reading range?

    - How does the modulation depth affect the reading range?

    - I've read that the antenna should not be tuned to exactly 13.56 MHz to prevent saturation. What is saturation about?


    Cheers.

  • 0 in reply to Jose Maria Fernandez
    Expert 1130 points

    Hi Jose; your Network Analyzer set up and your 6x6cm Loop (aerial) may answer my curiosity on the matter of detuning of the Loop aerial (a 13.56Mhz parallel resonant circuit). Please show a sequence of analyzer images as you place (stack) 1 then 2 then 3 ... right on top of your pcb "loop aerial".  My curiosity relates to detuning of parallel resonant circuits in close magnetic coupling . This matter in turn relates to a possible cause of "false or failure to read some Tags", more so larger than smaller tags as you have in mind to use. Thanks in advance.

    Ray

  • 0 in reply to Jose Maria Fernandez
    TI__Mastermind 44795 points

    Jose,

    Please see my answers in red below.  3cm will be fairly difficult to achieve with the RF37S114.  We recently did some testing of different antenna sizes and range with the RF37S114 and the max we saw was around 1.6cm(app note to be released shortly).  I suggest trying to increase the Q, but going too high can also cause slicing off the sidebands.  20 is typically the max, but you can test with higher Q and also observe for consistent communication.

    - Selecting a data rate of 1.65 kbps instead of 26.48 kbps improves the reading range?  The lower data rate could improve the range, but it will not be significant.  I suggest testing both data rates.  I would not expect more than 5% increase at best and likely smaller than this.

    - How does the modulation depth affect the reading range?  Modulation depth is specific to the tag IC that you are using.  For TI ISO 15693 tags, including the RF37S114, I recommend using 100% modulation depth for optimum performance.

    - I've read that the antenna should not be tuned to exactly 13.56 MHz to prevent saturation. What is saturation about?  I assume that this is talking about saturation of the receiver.  You don't want to over saturate the receiver or there will be too much noise on the receiver and it cannot decode the data.  In the case of TRF79xxA, this should not be a concern unless you are also using an amplifier.  In one of our old amplifier app notes, there was a mention of offsetting the tuning to ensure the receivers are not saturated, but in typical use cases, this is not a concern.  Generally, it is best to tune to 13,56MHz, but you could also try to go a little higher to ~13.7MHz to reduce effects of mutual detuning between tag and reader antenna.