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LMX2572: FSK Deviation (MHz) with high PLL_DEN

Malo Mabon
Prodigy 35 points
Part Number: LMX2572

Hello,

We like to use the PLL LMX2572 in FSK mode. We use the software TICS Pro in order to configure the eval board LMX2572EVM.

We want to program a jump of order of MHz around a fixed frequency.

Following the equation given in the LMX2572 datasheet (equations 9 & 10 p73), we succeed to have those jumps with quite small PLL_DEN (<100M) but with higher values it does not work.

We don't see any limitation on the LMX2572 datasheet.

Here are our PLL configuration in TICS PRO in order to reproduce the problem:

  • OSN_IN = 100MHz
  • Fpd = 100MHz
  • PLL_N = 60
  • PLL_DEN = 4294967295
  • PLL_NUM = 0
  • PFD_DLY_SEL = 5
  • MASH_ORDER = 4th Order Modulator
  • MASH_RESET_N = enable
  • Fvco & RFoutA = 6GHz
  • Output MUX A = VCO
  • FSK_DEV_SCALE = 13
  •  FSK_SPI_FAST_DEV = 1311
  • FSK_MODE_SEL = FSK_SPI_FAST
  • FSK_EN = enable

Is there a reason why we can't observe a deviation of MHz when we have a high PLL_DEN ?

Thanks in advance for your answers.

Best regards,

Malo

  • 0
    TI__Guru* 85330 points

    Hi Malo,

    We will check this out in the lab.

  • 0
    TI__Genius 14781 points

    Hi Malo, 
    Given the that FSK_DEV_SCALE is limited to 3 bits we have a limitation. 

    To get around this we can reduce PLL_DEN value to 2^24: 

    I was able to successfully see a 2MHz modulation by setting FSK_DEV_SCALE = 5 & FSK_SPI_FAST_DEV = 10,486 and then setting FSK_EN = 1

    Regards, 

    Vicente 

  • 0 in reply to vicente flores prado
    Prodigy 35 points

    Hi Vincente,

    Why is FSK_DEV_SCALE limited to 3 bits ? On the R115 register description p61, we can see that FSK_DEV_SCALE takes 5 bits on this register (maximum value seems to be 31).

    With PLL_DEN = (2^32)-1, we were not able to see a deviation of 500kHz with FSK_SPI_SCALE = 11 & FSK_SPI_FAST_DEV = 10486. Instead we see, a deviation as if  FSK_SPI_SCALE = 10 (250kHz).

    It seems the maximum FSK_SPI_SCALE is 10 (higher values are ignored).

    With PLL_DEN = 2^24, we can reach deviation of 24MHz (with FSK_SPI_SCALE = 7).


    We don't see any limitation for FSK_SPI_SCALE in the LMX2572 datasheet.

    Regards.

  • 0 in reply to Malo Mabon
    TI__Genius 14781 points

    Hi Malo, 
    My mistake - you're correct FSK_DEV_SCALE is 5 bits. I read the register length incorrectly. 

    What frequency deviation are you trying to achieve? That way I may better assist you.  I understand you required a 6GHz RF output with a 2MHz deviation. 
    Setting FSK_MODE_SEL = FSK SPI FAST ;  PLL_DEN = 2^24 with FSK_DEV_SCALE = 5 ; & FSK_SPI_FAST_DEV = 10,486 and then setting FSK_EN = HIGH will result in a 2 MHz deviation. 

    Regards, 

    Vicente 

  • 0 in reply to vicente flores prado
    Prodigy 35 points

    Hi Vincente,

    We don't want any specified deviation or carrier frequency, we just want to build a generic driver that can do a FSK modulation as it is described in the datasheet.

    A high PLL_DEN require high FSK_SPI_SCALE but we observed the PLL calculation of deviation take a limited value instead of the define value.

    For our exemple we would like a 500kHz deviation, we set:
    - PLL_DEN = 2^32-1
    - FSK_SPI_SCALE = 11
    - FSK_SPI_FAST_DEV = 10486
    - FSK_MODE_SEL = FSK_SPI_FAST
    - FSK_EN = Enabled

    What we observed is a 250 kHz deviation. Which is what we supposed to have with FSK_SPI_SCALE = 10. So we wondering about a limitation.

    Regards,

    Malo

  • 0 in reply to Malo Mabon
    TI__Guru* 85330 points

    Hi Malo,

    it does not look straight forward to me, I can check this out in next week. 

  • 0 in reply to Noel Fung
    TI__Guru* 85330 points

    Hi Malo,

    I checked this out, right, looks to me that it is limited to =10. I am checking this out with the designer, I will get back to you later.

  • +1 in reply to Noel Fung
    TI__Guru* 85330 points

    Hi Malo,

    it is confirmed that FSK_SPI_SCALE is in fact limited to 10, after 10, they are actually equal to 10.