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Using PPI to Generate 8MHZ Signal

levijo

Hi,

I'm using the PPI component to generate 8MHz GPIO signal. this signal is an input for power amplifier analog RF transmitter.

While the digital 8MHz signal looks fine, we are noticing some bumps every 40 us in the RF analog signal. when trying to use the SPI clock to create the 8MHz signal the bumps are eliminated and we get a clean signal.

As the PPI input clock is 16MHz, I was wondering if theoretically its capable to create a digital 8MHz signal? that means that the GPIO should be toggled every single clock, is that an overkill in terms of performance for the PPI or should it be able to handle this task?

The bumps were also reproduced with the DK evaluation board to generate the 8MHz signal, see bump in the image below:

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  • 0

    Hello Joshua,

    I'll have to get back to you on this tomorrow.

    You are using the 5340?

    Are these bumps a problem for your application?

    Regards,

    Elfving

  • 0 in reply to levijo
    Elfving said:
    so getting what you described above working here would be great. It seems I don't have the right components in the lab right now, but I'll try to test this tomorrow.

    I see that I unfortunately won't be able to test this today after all. I'll have to try testing this after the holidays.

    AvihayZ said:
    Zooming in to the rectified signal will show the 8MHz ripples after the envelope detector. Are these the oscillations you mean?

    Yes. I'm not familiar with envelope detectors myself, but I figured it could tell us a bit about this other signal that is in the mix. Though I am a bit confused if the "steps" are from the bumps that we're trying to figure out. Then there are two things here that are inexplainable, right? If in fact the signal from the nRF52 was completely smooth.

    AvihayZ said:
    In the meantime, can you please share your opinion whether it might be related somehow to the operation of the PDM module

    It definitely could be related, especially considering what you mentioned here.

    levijo said:
    3. When activating the Dmic with nRF53 the output signal have a significantly less bumps (every few seconds instead of every few ms)

    Could be that enabling it makes some changes regarding consistent clock management, resource allocation, or interrupts. Though as you said, you are still seeing the issue with this enabled. It isn't necessarily better per se. Getting this reproduced on my side would make things easier.

    Regards and happy holidays,

    Elfving

  • 0 in reply to Elfving

    Hi Elfving,

    Just to give you some more inputs:

    1. we have changed the code to generate the 8MHz via SPI (see attached code), using the nrf5340 dk board.

    2. regarding the DMIC, we disabled it entirely to see if it has an effect on the bumps. The output signal still remained with the bumps

  • 0 in reply to levijo

    Hi Elfving,

    one more input is that NFC is turned off in our case

  • 0 in reply to levijo

    Hi again Joshua, and happy new year!

    I'm trying to recreate what you are seeing with the envelope detector, but I am not seeing it yet. Although I am not using a schottky diode, did you arrive at that being necessary?

    Could you show me how the output of the envelope detector looks on your side with a 52? I assume that looks as smooth as the signal generator?

    Regards,

    Elfving

  • 0 in reply to Elfving

    Hi Elfving,

    Did you try to reproduce it on the Nrf52? we don't see the problem in this board so maybe try to reproduce it on the nRF5340 instead

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