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TRF7962A - Antenna Matching Network

Other Parts Discussed in Thread: TRF7962A

I'm using the TRF7962A on my PCB with the recommended 1.5uH inductor antenna + RLC network.

It works well with tiny 0805-sized tags at 0.3", but drops out after that.

To get more range I'd like to use the FXR.05.A, a larger 15.9uH antenna.

I used the equations provided in slou372a with Q=20 to calculate Cres=8.7pF, Rpar= 27.4K.

I downloaded Smith v3.1 but it won't allow me to add the RLC network until I chose a point on the chart.

Can someone please explain how to use this tool or provide a TI Smith file? Or give equations for the matching capacitors?

  • nick -

    i think you might be going a direction which leads you to nowhere.

    adding inductance or size won't help you because the tag is small - meaning: it is the bottleneck here because the smaller the transponder, the more dense of a field it needs to be activated and the faster its response will fall off, over distance.

    in fact - adding inductance will lead you to a place where you cannot tune the antenna and making it larger in size will make the field larger, but weaker overall.

    are you tied to that tag size? would you consider a 4mm x 4mm transponder device (we have one about to come out to the market)

    its a little more than twice as big as what you have today.

    also, what is orientation you are using? in the 0805 package, i would expect there to be a ferrite core tag antenna inside, which would make it directional - so then you might be better off making a ferrite core reader antenna to interact with it.

    as far as Smith Chart help goes -

    if you have inductance and can make an assumption that the coil itself will be in the 1-2 ohm range, then to use the tool, you just calculate the imaginary part (2pi*f*L) to enter the starting point for matching.

    for example, 1.5uH...(assuming 1 Ohm of R) would give you R + jXL = 1 + j127.7352)

    if you measure the coil at frequency using network analyzer first, then you don't have to assume the R value 

    this you would enter in with the Keyboard data selection input from top left side of the screen. (and set your default frequency of the tool to 13.56MHz)

    then if you multiply the jXL value by 16, it will give you 2kOhms to use for Q dampening. This will be a starting value - you will most likely need to adjust it to a standard value below it (like 1.8k) for final match.

    so that would look like this:

    hit OK and you have first data point on the Smith Chart

    then choose standard values that get you where you need to be - like this: (and it turned out 1.8k shunt was OK choice if just one standard value cap was used on shunt) - like i said, this will get you close enough to populate your circuit and then measure on network analyzer to then fine tune some direction in pF and Ohms) 

    and come back for more help if needed. 

  • Josh,

    Thank you very much for the info. We're not an RF company and don't own a Network Analyzer, so we really appreciate your advice.

    We're very space-limited.

    A. We use wirewound inductors as antennas:

    BRL2515T1R5M (horizontal orientation)

    SDR0805-1R5ML (vertical orientation)

    B. We use this transponder: LXMS33HCNG-134 

    We'd like to know:

    1) Which small RF antennas or inductors do you use?

    2) What's the PN for the new 4x4mm tag? Are samples available? Will it be in production by Sept-Oct?

    3) Which other small tags / transponders do you use? What range do you get with them on your bench?

  • Nick - 

    the Murata tag part number (LXMS33HCNG-134) has the antenna embedded inside of it (i am familiar with it - as you can see on their data sheet, they used our reader EVM with it). So, are you using a different part number perhaps? 

    next I would ask you about your inductor selections and how you are using them - these are only 1.5uH in value - the Bourns device is tested at 7.96MHz, the Taiyo-Yuden device is tested at 1MHz and both have a tolerance of +/-20%

    then are you attempting to couple in the reader signal with no physical connection into the tag? is that correct? it just doesn't quite make sense yet as you called out the complete tag already.

    regarding the 4mm x 4mm - please send me email directly to discuss samples and timing, etc (josh.wyatt@ti.com)\

    thanks! 

  • Josh,

    1) The Murata tag is the correct PN. As you know, its a complete transponder including antenna. Why do you think we wouldn't use this one?

    2) We selected 1.5uH inductors (0.02 / 0.13 ohm DC), which had similar inductance to the 0.95uH EVK loop antenna. What inductance / DC impedance do you recommend?

    3) The reader antennas will be on a customer PCB, with the tags up on metal contactor blocks, at 90° to the PCB. At a max of 0.3-0.5" away.

    Thank you.

  • OK - now its making sense - you are using the coils for the reader side. sorry it seemed like you were trying to use the for tag side (which of course already has antenna built in)

    the problem with these is that they are 20% devices.
    this means that you would end up potentially having some populated which were between 1.8uH to 1.2uH.
    this causes a problem because you want each part of the resonant circuit to be as fixed as possible, so its repeatable.

    for example:
    you do a resonant match for 1 + jXL and you have calculated XL (based on L = 1.5uH) to be 127.7352
    but, then you have some which are actually 1.8uH or 1.2uH, which would then be XL = 153.2822 and 102.1882, respectively.

    this means then that your dampening resistor, series and shunt caps would all be different to reach same correct resonance / impedance point.

    so long explanation to the simple point that high tolerance parts need to be used here. (to put in perspective, this is not switching power supply (where you use those kinds of tolerances and get away with it, its a tuned circuit)

    the G tolerance of these might be a good ones to look at for the one dimension ==>
    www.abracon.com/.../AISC-1008(F).pdf
    www.abracon.com/.../AISC-0805(F).pdf

    this is also a cool tool that coil craft has for finding inductance @ frequency
    www.coilcraft.com/.../inductancefinder.cfm

    now the other one might be a little harder to cross - i will keep looking.
  • Thanks for your feedback.

    We selected the AISC-1008F-1R5G-T, and updated the matching network. PCB is in fab.

    We appreciate you looking for a vertical inductor. We'll keep looking also. So far nothing in GHJ tolerance / 0.5-10uH.

    If we can't find one, we'll spin an antenna PCB with the SDR0805-1R5ML. We understand that performance will vary, but proto is only $5.

  • After some digging, the only 5% vertical inductor I could find is the LQH43NN100J03L. Its 10uH / 560mOhm RDC.

    Its a pretty high L, and 13MHz is close to the 23MHz self-resonant freq. 

    With this one, the L is the resonator and it doesn't really need a parallel cap. 

    Its worth a $2 experiment. We'll let you know how it compares. Thank you again for your help.