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CC2650EM-4XD-RD: Carrier frequency offset measurements

Eddie Koh
Genius 10505 points
Part Number: CC2650EM-4XD-RD

Customer's board followed CC2650EM-4XD-RD design. 

At measurement, the following carrier frequency offset was observed:

CC2650EM-7ID - 5.5KHz

Customer's board (based on CC2650EM-4XD-RD) - 65KHz

It was noted that both are well within the 150KHz limit.

Question: Customer would like to understand what is the key factor (e.g. caps, inductors, crystals, layout, power supply, etc..) that could impact carrier frequency offset, so that they can improve their design to match TI's design.

  • 0
    TI__Guru** 106705 points
    Hi Eddie,

    The carrier frequency offset depends solely on the frequency offset of the 24 MHz crystal oscillator. This again depends on the parameters of the crystal, layout and the load capacitors. The latter are internal on the CC26xx devices, and are configured in CCFG.

    In other words, improving the offset is just a matter of adjusting the load cap settings. This wiki article has more details: processors.wiki.ti.com/.../CC26xx_Tips_and_Tricks

    Best regards,
    Fredrik
  • 0 in reply to Fredrik K
    TI__Genius 10505 points

    Hi Fredrik,

    We followed the steps in Wiki link to change the cap-array delta, but found that the carrier frequency offset did not change even when we applied the extreme values of -27, 0 and +10.

    Is there any other way to alter the carrier frequency offset? 

    Attached image is the code modifications made to change the cap-array delta.

  • 0 in reply to Eddie Koh
    TI__Guru** 106705 points
    Hi Eddie,

    That sounds strange, you should see a noticeable difference. The settings look correct. Are you sure you compiled and programmed properly after each change?

    Best regards,
    Fredrik