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CC3220SF: CC3320 and 125KHz Receiver

Chris_CBC
Intellectual 470 points
Part Number: CC3220SF

We have the CC3220 working well from a digital and WiFi standpoint in our product. Where we are having a problem is with noise, and specifically 125KHz noise.

Our product has a 125KHz receiver. We are finding it is being deafened by noise from the CC3220. We found that one of the internal switchers of the CC3220 was one of the culprits (DCDC_DIG_SW), but we have isolated that noise source and we still have problems. We think we have eliminated all other noise sources on the board (all other active devices have been removed), but we still have significant noise problems with the AS3933. We are wondering where the CC3220 might be generating this noise and how we could eliminate it.

  • 0
    Guru 73375 points

    Hi,

    That is interesting. For TI support may to be interesting if you will provide measurements from spectrum analyser (results + test setup). Can you test with 1.85 V pre-regulated mode?

    Jan

  • 0 in reply to Jan D
    Intellectual 470 points

    Spectrum - I don't have a picture, but I can describe my observations - When the DCDC_DIG_SW is on we see a 1 to 1.5MHz switching frequency that is being turned on for a short bursts at about a 70KHz rate. In the passband of the receiver this puts a fundamental tone at 70KHz and a second harmonic at 140KHz. The 70 and 140KHz spurs can be moved by increasing the load on the DCDC_DIG_SW switcher. More load equals higher frequency for the 70 and 140KHz spurs. This I was hoping would be a solution to the problem, but as we shift out one tone out of the passband the other tone shifts further into the passband. We got rid of these spurs by inserting a voltage of 1.2 volts to the regulated input/output of the switcher. There was a noticeable improvement in noise level seen at the output of the LF receiver when this was done, but there is/was still a lot of noise at the output.  

    Test Setup - The board is an advanced prototype PCBA with a number of peripherals including an accelerometer, temperature sensor, and LF receiver. There are a Qi wireless power receiver and battery charge controller as well, but these are powered down during LF receiver operation.

    We have considered using the pre-regulated mode, but at the moment it is impractical. The LF receiver requires a minimum of 2.4 volts to operate. The insertion of the 1.2 volts at the DCDC_DIG_SW seems to simulate the affects of using the pre-regulation mode anyways.

     

      

  • 0 in reply to Chris_CBC
    TI__Mastermind 34850 points

    Hi Chris,

    Please request a schematic/layout design review using this page. Include the BOM as well.

    BR,

    Seong

  • 0 in reply to Seong Kim
    Intellectual 470 points

    Seong,

    Is this review private or will it be visible to the forum?

    How long does the review take?

  • 0 in reply to Chris_CBC
    TI__Mastermind 34850 points

    Hi Chris,

    It will be private. Once the review is assigned, it can take up to 4 business days.

    BR,

    Seong

  • 0 in reply to Chris_CBC
    Intellectual 470 points

    Seong,

    I have summited the review request. Is there anything above that we can start discussing as part of the troubleshooting of the noise issue?

  • 0 in reply to Chris_CBC
    TI__Mastermind 34850 points

    Chris_CBC,

    I reviewed the design and as mentioned in my feedback, the three traces that serve as return current paths on layer 2 is required to isolate the RF ground from the noisy supply ground and to reduce EMI. This routing must be strictly followed because it is critical for the overall performance oft he device.

    BR,

    Seong

  • 0 in reply to Seong Kim
    Intellectual 470 points

    Would this explain why we would be seeing noise in the  LF receiver at 125 KHz?

  • 0 in reply to Chris_CBC
    TI__Mastermind 34850 points

    Hi Chris,

    Let's keep this discussion on email for now.

    BR,

    Seong