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CC1200 VCO Frequency Programming?

MikeH
Guru 15580 points

I am trying to understand how the values of FREQ0, FREQ1, and FREQ2 are derived. I want to operate in the 900MHz band (902 - 928MHz). As I understand from the User Guide, the desired RF frequency is 4 x the VCO frequency (for this band). Therefore the VCO will operate a 902 x 4 = 3608MHz. If I convert 3,608,000,000 to hex, I get 0xD70DB600. How do I fit this number into three 8-bit registers?

I am obviously doing something wrong since I can't correlate the values generated by Smart Studio to my desired frequency. Any guidance would be appreciated.

  • 0
    TI__Guru**** 317180 points

    Note that the FREQ word is given by equation 27 and 28 in the UserGuide. Solving the two equations with respect to FREQ you get.

    FREQ=fRF*LODIV*2^16/fxosc

    Using this you will get the same as SmartRF Studio.

  • 0 in reply to TER
    Guru 15580 points

    TER,

    Thanks. Your equation works. However, the User Guide is a bit confusing. I thought the second equation was an optional way to program the VCO since the words in the paragraph say "There is also a possibility to perform VCO frequency offset programming". 

  • 0 in reply to MikeH
    TI__Guru**** 317180 points

    As you see in section 9.12 of the UserGuide the VCO frequency is a function of the FREQ word and the contents of the FREQOFF register. As explained in the paragraph the estimated frequency offset could be transfered to the FREQOFF register with a SAFC strobe or if you have a fixed offset in the system (typical if the load caps to the crystal is a bit larger/ smaller than they should be) you can cancel this offset by setting the FREQOFF manually.