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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 Hung Bui

    Hi,

    1. the digital signal looks fine, we couldn't find jitters, but we suspect there are some and we can't capture it with our scope, as we managed to get a signal without bumps when using the SPI clock for the 8 MHZ instead of PWM without the bumps, so we suspect there must be some jitters.

    2. we haven't tried to use another GPIO, are there some GPIO's that are more recommended to use than others?

    3. The code I have provided is a minimal code that produces the digital using PPI and PWM, other than that nothing is configured to be enabled, unless there are some peripherals that needs to be shut down actively that I'm not aware of.

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

    5. We have managed to configure the PWM with the nRf52 MCU in our previous HW design and it worked fine (Note that our current RF design is not exactly the same), what clock would you recommend using for RF communication? 

  • 0 in reply to levijo

    Hi again, 

    Some GPIO are closer to the radio internally and I was thinking that the radio activity may affect the GPIO. But as you mentioned you reproduced the issue without other peripheral active so it could be something else. It's worth to try on different GPIO pin though. 

    8MHz is about at the maximum limit of PPI&TIMER. Our peripheral is running at 16MHz and in your case it need 2 cycles to toggle up and down the GPIO to generate the clock signal. SPI uses the HFCLK to generate it SCK signal so it's more stable and more reliable from my point of view. 

    Please let us know the result of your tests. 


  • 0 in reply to Hung Bui

    Hi,

    see below some more inputs

    1. we tried a different GPIO, the results are the same - bump occurs in the final signal

    2. we tried using nRF52 instead of nRF53 to the same RF analog input, we get a clean signal with no bumps

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

  • 0 in reply to levijo

    Hi again Joshua,

    So the signal from PWM sample you sent me looks ok on the nRF53, though I see that there is some additional jitter there at 13.5Mhz in addition to 8Mhz and 16Mhz for some reason.

    I do not see the same spike on the nRF52. I don't know what that is doing there yet.

    Regards,

    Elfving

  • 0 in reply to Elfving

    Could you expand a bit on what the power amplifier analog RF transmitter is doing? Is the rf signal coming from the nRF in question? Is the pwm signal working as a clock for the PA?

    Have you tried with hw pwm?

    Regards,

    Elfving

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  • 0 in reply to Elfving

    The 8MHz signal produced by the PWM serves as the transmitter's carrier frequency. This 8MHz signal, generated by the nRF, is sent to the power amplifier (PA) in bursts lasting several hundred microseconds. These bursts generate an electromagnetic field via an antenna connected to a resonant circuit, enabling the transmission of wireless power.

    Any deviation from 8MHz affects the resonance circuit tuning and reflected in the transmitted signal.

    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?

  • 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?

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