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  • Case ID: 335301
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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
    levijo said:
    In a preliminary emission tests we conducted on this board, we indeed saw some high harmonics of 13.5MHz. Is that something you can check why this spike at 8MHz is added to the signal only in nRF5340? Is it possible to avoid it somehow? Is there something in the example code I attached that can explain this additional 13.5MHz?

    Not that I can immediately see, though I am not sure what that could theoretically be either way since I am not seeing it on an nRF52. I'm trying to check why that is there.

    Have you checked if the time between these bumps change depending on the frequency?

    Did you see if you see these bumps with a regular pwm peripheral? The pwm peripheral wouldn't be able to get you 8MHz though, so I would understand why you haven't tried this. 

    Regards,

    Elfving

  • 0 in reply to Elfving

    Hi Elfving,

    There is no evidence that the 13.5 Mhz signal is related to the bumps. both the frequency (77ns vs 40 us) and the signal amplitude doesn't explain it.

    We are using the zephyr api to generate the signal (which is approved by Nordic). 

    Can you spot anything in the digital 8MHz that can explain the bumps?

    Can you give us a sample code you recommend using to test 8Mhz signal on specifically? or even using the PWM directly (with the highest frequency possible)?

  • 0 in reply to levijo
    levijo said:

    Can you spot anything in the digital 8MHz that can explain the bumps?

    No I can't, nothing besides the 13.5MHz part. Though I agree that this doesn't really explain anything regarding the bumps. Although it is to me what stands the most out here. I've asked an internal team with help investigating what this could stem from, though the obvious NFC association comes to mind.

    levijo said:
    Can you give us a sample code you recommend using to test 8Mhz signal on specifically? or even using the PWM directly (with the highest frequency possible)?

    I'll have to get back to you on the latter part of that next week, but what do you mean in regards to test the 8MHz?

    Regards,

    Elfving

  • 0 in reply to Elfving

    Hi Elfving,

    we have done some more tests to find out the source of those bumps, we have used an “envelope detector” (see drawing below) along the signal path and analyzed the pulses at each point.

    D1 - 1PS76SB21 (can use another P/N)

     

    We have found that even when connecting this “envelope detector” to the MCU output pin that provides the 8MHz signal, there are evidence that the signal is not consistent and there are steps every 40-60uS

    The output signal (@Vout1):

    When we connected the “envelope detector” to a 8MHz signal from a signal generator the pulse was smooth. The conclusion is the source of the bumps is from the MCU,

    Can you please try to reproduce the test we have done and try to find the cause of this 8MHz signal bumps?

  • 0 in reply to levijo

    Hi Joshua, 

    Sorry about this case going on as long as it has. My problem currently with this is that we have yet to reproduce this on our side (at least if we do not think this 13.5MHz problem is related), 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.

    Could you send me a higher resolution version of that image from the scope? So, the envelope detector there seems to show some oscillations regarding the peaks, could you show me what it is oscillating between? And in addition to that it seems to show that the average gradually goes up a few steps there, do you agree? Is that also inexplainable from your side? 

    levijo said:

    4. We will try to lower the frequency and see if it reproduces

    Did you get a chance to test this?

    So on the graph here, what are we seeing in brown, blue and cyan?

    Regards,

    Elfving

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  • 0 in reply to levijo

    Hi Joshua, 

    Sorry about this case going on as long as it has. My problem currently with this is that we have yet to reproduce this on our side (at least if we do not think this 13.5MHz problem is related), 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.

    Could you send me a higher resolution version of that image from the scope? So, the envelope detector there seems to show some oscillations regarding the peaks, could you show me what it is oscillating between? And in addition to that it seems to show that the average gradually goes up a few steps there, do you agree? Is that also inexplainable from your side? 

    levijo said:

    4. We will try to lower the frequency and see if it reproduces

    Did you get a chance to test this?

    So on the graph here, what are we seeing in brown, blue and cyan?

    Regards,

    Elfving

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