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CC1312R: Radio frequency calibration for 48MHz crystal

Part Number: CC1312R

I am looking for the best way to calibrate the 48MHz external crystal for each device (in production) for accurate radio frequencies in ISM band.

I see different calibration methods and wonder about their accuracy:

1) Measure frequency of a carrier wave at a specific frequency generated by the unit under test using a spectrum analyzer. Get the offset and apply it into command CMD_FS everytime we use the command.

2) Cap-array tuning: is there a formula for cap-array tuning based on frequency offset or we have to scan through multiple cap-array configurations until the output frequency is good. Also, TI's document mentioned using CCFG.c to apply cap-array tuning value, that will require project re-compile. How do I apply cap-array tuning value in runtime or using a Flash programming adapter?

3) Configure CC1312 to add frequency offset byte into received packets. Use that offset for calibration.

4) Use timer to measure actual frequency of 48MHz crystal in firmware.

Could you comment on the accuracy of each method and which one would you recommend?

Thank you so much!

  • Hi,

    When using the same crystal and board layout, the frequency offset should be very repeatable. Using the internal cap array to tune the crystal is the optimum method without using any external loading capacitors.

    The cap-array tuning of the crystal is documented in Section 6 of the SWRA640 app note: https://www.ti.com/lit/swra640

    In order to tune the crystal, the best approach is to measure the RF frequency offset. Transmit an unmodulated Tx carrier at a specific frequency (i.e. 915.000 MHz), then measure the frequency offset relative to the specified frequency. If measuring 915.002 MHz, then the frequency offset is 2 kHz which corresponds to 2.2 ppm for the crystal.