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TRF7970A Low Power gives higher received signal strength (RSSI)?

Stephen Hewitson
Intellectual 305 points
Other Parts Discussed in Thread: TRF7970A

Hi,

I've got a TRF7970A EVK board connected to a coil through the SMA connector, and I am using the RSSI level in Reg0x0F to evaluate coil read range. I've placed an RFID tag in the centre of the coil and I am monitoring the RSSI level.

When running at Full power, my Main and Aux RX strength are 5 and 0 respectively.  Using Half power, my Main and Aux RX strength are both 5.

This seems counter-intuitive, so what else is going on?

I guess that if my coil is does not have a characteristic impedance of 50ohms, then the 4-50ohm impedance matching circuit will behave differently - giving an overall better power transfer when the output impedance is 8ohms? What do you guys think?

Stephen

  • 0
    TI__Guru* 91206 points

    Stephen -

    You did remove R3, corrrect (to disconnnect the onboard antenna ckt?) if not that could be one area to take care of and then also, when you say your coil - you do mean your tuned coil, correct? (meaning you are providing your own series and shunt elements, with a Q factor appropriate for the tag technology you are using)

     

  • 0 in reply to Josh Wyatt
    Intellectual 305 points

    Josh,

    R3 has been removed.

    I measured the coil in isolation to have an impedance of 3+114.8j. Used a shunt resistor of 750R, shunt cap of 69pf, and series cap of 39pf. Then I connected the coil through a Skyworks switch, so I can connect multiple coils. I should have a Q of 6.5 and a bandwidth of 2.08MHz.

    I've measured the tuned coil through the switch to have a characteristic impedance of around 44ohms. I've got a transmission line that goes through the switch and then can get terminated with my calibration load - that gives |z| of 54R.

  • 0 in reply to Stephen Hewitson
    TI__Guru* 91206 points

    I agree that you R should be around 750 ohms here for that Q factor, however, i plotted out your tuning solution on the Smith Chart and i think you should adjust it the other way round.

    your shunt cap (in parallel with the R and the coil) should be 39pF and the series element should be 68pF. pls.check that out.

  • 0 in reply to Josh Wyatt
    Intellectual 305 points

    Josh,

    Sorry about that, I meant I had series cap of 69pF and a shunt cap of 39pF like you've shown.

  • 0 in reply to Stephen Hewitson
    TI__Guru* 91206 points

    Right - so the best to do - if you have a power meter or spectrum analyzer - is to hook it up (using attenuators if needed by the instrument so you don't damage it) and measure like what i have shown in this document (using the Peregrine digital switches) and see if you can get some more data about what is going on with your ckt.

    http://www.ti.com/litv/pdf/sloa167 (section 5)

    i have built several of these units and been able to replicate good performance and get back expected RSSI values.

    See video here of the smaller four channel version here ==> http://www.youtube.com/watch?v=16VFU9n8VO4&feature=plcp

  • 0 in reply to Josh Wyatt
    Intellectual 305 points

    Josh,

    I've got a PSA1301T 1.3GHz Spectrum Analyzer. Its a handheld unit, but I dont know if it is any good.

    It's got a max input power of +20dBm and ±15VDC. I dont have any attenuators, but if I run the TRF7970A at 3v3 and half power then I should be OK (14dBm).

    In which case do I just connect the spectrum analyzer to the output port of my switches?

    EDIT:

    I didnt want to blow anything up, so I am using a near field prob instead. I've positioned the probe near one of the coils. I can see a spike at 13.56 MHz, at approx -22dBm for Half Power, and -19dBm for Full Power.