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CC2340R5-Q1: How to Tunning Load capacitance to get more precise Frequency

QL
Prodigy 130 points
Part Number: CC2340R5-Q1
Other Parts Discussed in Thread: SYSCONFIG, CC2340R5

Hi Ti teams,

    Now we are using CC2340R5-Q1 to design for becoming a bluetooth board.  We will do some RF test to get certification.

    One of the test item is Frequency deviation, but the performence did not meet the request. Now we send the CW signal to test Frequency deviation in 2440Mhz, and get the Frequency error is 125Khz as the picture below. And Crystal we use right now is 48Mhz.

    

    And How can we tune to get more Precise Frequency, and the certification request Frequency accuracy limit is (-20,20)ppm, which is about (-50, 50)khz. Now the frequency acuuracy of our dut is performing out of the limit.  And how can we tunning to get better one?

    Here is our SCH and if we wanna tuning the internal Load capacitance in CC2340R5-Q1, If there is any bad influence? Or What we can do is only tunning the external Load capacitance on boards?

   Best Regards,

   Annie

    

  • 0
    TI__Mastermind 39800 points

    Hey Annie, 

    As a heads up, much of TI in the US and EMEA are on a holiday, so feedback here may be delayed a bit. For tuning the HF XTAL, you can either use external load capacitors or leverage the internal cap array which can be modified in Sysconfig under device configuration. You will see a section called HXFT Cap Array where you can adjust the values. 

    The below app note for a different generation of devices also has some advice and is largely a similar process with the CC2340R5 to an extent. 

    https://www.ti.com/lit/an/swra640g/swra640g.pdf

    For the LFXT, you should tune it via the external load caps. 
    Looping in someone from my team who can assist you further from here on. 

  • +1 in reply to Evan W
    TI__Intellectual 2490 points

    Hello Annie,

    I believe the app note Evan provided above should provide you with the guidance needed to tune the HF XTAL using the internal cap array. When using the internal cap array for the crystal load capacitance, we recommend only using the internal cap array. It appears your design also includes external caps which should not be populated when using the internal cap array.  Tuning the internal cap array should allow you to achieve a frequency accuracy well within +/-20ppm at room temperature.

    For the LFXT, section 6.3 of the swra640 app note Evan linked above provides guidance on how to calculate the load caps required for a given crystal with a specific load cap value. Based on your schematic image, you've selected a crystal that requires 12.5pF of load capacitance which is higher than our maximum allowable load capacitance per our datasheet.

    Please select a different crystal that requires a load capacitance between 6-12pF per our crystal requirements detailed in the datasheet. For a list of recommended crystals, please refer to app note SWRA495  section 6. We typically like to use crystals that require a load capacitance of 7pF. With a 7pF load capacitance crystal, the resulting load cap value would be 12pF (12pF/2+1pF (parasitics) = 7pF). This means you would not need to change the capacitors in your schematic and would only have to change the crystal. 

    Also, if you want to measure the accuracy of the LFXT, section 6.3 of the swra640 app note Evan linked above provides guidance on how to do that as well.

    I hope this helps.

    Andrew

  • 0 in reply to Andrew Lugo
    Prodigy 130 points

    Hi Andrew,

     

        Thanks for your help, it help me a lot about how to choose suitable Crystal for Device! But abot the internal capacitance, we need know more detail and choose one sulution, and I will ask Evan about more detail about tunning internal capacitance, Thanks a lot!

        Best Regards,

        Annie

  • 0 in reply to Evan W
    Prodigy 130 points

    Hi Evan,

        Thanks for your kindly help. And Sorry for disturd you in your holidays.

        We still have something confused about that. So We wanna know more detail about the internal capacitance. One thing we need to confirm is that how can we ensure that HXFT Cap Array which we tune value will be applied, it will be OK in Host test FW. But later we will manufacture the products to our customs, whose FW will be not Host test, Maybe it will be basic ble FW or Others. We are afarid that HXFT Cap Array will disappeared when we fashed other FW.

        Hope for your reply, Thanks!

       Best Regards,

       Annie

  • +1 in reply to QL
    TI__Mastermind 39800 points

    Annie,

    You set these values in Sysconfig for which ever example you base your project off of. Each project has a sysconfig and you can modify these values there under Board Configuration to your desired cap array values based on what you have measured is right for your application.