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Original question:

Frequency of the auto-calibration of the CC1101 RF radio chip ??

CC1101: Need to perform a production FREQ2..FREQ0 calibration on a CC1101, with a 20ppm crystal?

Rich Mullen
Intellectual 325 points
Part Number: CC1101
Other Parts Discussed in Thread: CC1190

I have a custom, CC1101/CC1190, 869.525 MHz board and I'm using the NDK NX3225GA 27MHz crystal, same as the one in the TI CC1101-CC1190 869 EVM. The crystal has a +/-20ppm tolerance, +/- 30ppm over temperature.

With FREQ2, FREQ1, FREQ0 set to 0x203462, the CC1101 center frequency should be 869.525 MHz.

The measured center frequency has consistently been 869.46x MHz. Multiple boards, two testing companies. Calculation with a 20ppm 27MHz crystal set the range to 869.507 to 869.54.

Fcarrier = (Fosc / 2^16) * FREQ

The question is what would be the best way to address this and get the center frequency back to 869.525 in production? Modify the FREQ2..0 values in an after production test, or am I missing something?

Thanks for for your time!

-Rich

  • 0
    TI__Guru**** 317180 points
    Have you looked into adjusting the load caps for the xtal? If you have different parasitics on your board compared to our evaluation boards you have to adjust the Cload.

    Or you are using a xtal that has a different Cload spec than what we use.
  • 0 in reply to TER
    Intellectual 325 points

    Thanks TER,

     Measuring the 27 MHz crystal, I got a value of 26.9971 and that does account for the lower center frequency. I used the same crystal and cap load that was on the EVM, though, I did ground the two extra pads on the crystal as the crystal data sheet says that is good practice. The EVM PCB didn't have them grounded, only attached and floating. I didn't think that one through.... Thanks for your advice!

      As we have a few hundred boards made, I put in a fixed adjust to the frequency multiplier that took care of the issue. The secondary question of this thread would be if it's typical, or a creates a problem, to perform a production calibration to adjust the frequency multiplier (FREQ0,1,2) to ensure the center frequency?

    -Rich

  • 0 in reply to Rich Mullen
    TI__Guru**** 317180 points
    For this, look into the FREQOFF[7:0] register. That is easier that recalculation the freq word.