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Clock requirements for TRF7970 13.56 MHz XTAL input

Other Parts Discussed in Thread: TRF7960

One of the wearable designs using the TRF7970 has space constraints and we're aggressively eliminating passives where possible.  Our next target: 3 passives used for clocking the 13.56 MHz Xtal pins (XTAL +2 caps).  We would like to replace the bulky crystal and load caps with a micro-power 1.5 mmx 0.8 mm oscillator  of the same frequency.

What should be the frequency stability and phase noise of the external 13.56 MHz external clock?

Thanks,

JP

  • Hey JP,

    Here is the published crystal requirements: 

    Honestly, I think the TRF is pretty robust and could work with a wide range of crystals.  The issue you are going to run into, is the crystal frequency is what drives the carrier freq.  Depending on the countries you plan on operating in, you will have to meet the regulations, such as FCC.


    I believe FCC specifies +- 100 ppm from -20 to 50C and in Japan it's +-50 for all temps.  

    Thanks,

    JD 

     

  • Hi JD,

    Thanks for the frequency stability requirements. How about the phase noise requirements if I'm driving an external clock(square wave) in to the TRF7590?  It would be good to get a phase noise table at various frequency offsets from the carrier:

    Frequency Offset

    (kHz)

    Phase Noise (dBc/Hz)

    1

     

    10

     

    100

     

    1000

     

    Thanks,

    -JP

  • JP -

    the worldwide spec for frequency deviation allowed is +/-7kHz of 13.56MHz

    the measurement we did was @ 10kHz offset, which was measured and then calculated to be 103.77dB, under those conditions

    see attached, slide 8

    TRF7960 Reader (Hardware Dsgn).pdf

  • Hi Josh,
    Thanks for sharing the measurements. Did you mean -103.77 dBc/Hz at 10 KHz offset is the requirement for a 13.56 MHz external clock in to the TRF7960? Looking at the spectrum plot on slide 8, this 13.56 MHz clock source hits a noise floor of -114 dBc/Hz at around 20 KHz offset and beyond. Can you please confirm if I'm reading this correctly? Also, is this true for a 27.12 MHz clock source?
    Thanks,
    -JP