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LMX2571: Clock & timing forum

Jun Mori
Prodigy 20 points
Part Number: LMX2571
Other Parts Discussed in Thread: LMX2572LP

We are trying FM modulation in the frequency band of 30 to 50MHz with LMX2571.

The FM deviation should be 5KHz.

It has the following settings.
Positive modulation is applied, but negative modulation is not applied.

Please tell me what kind of register setting should be done.

  • 0
    TI__Guru** 109350 points

    Hi Mori-san,

    PLL_DEN should be set to 2^24 (or 2^24 - 1 in the GUI) when FSK is used. 

    The noise contributed by the Multiplier will be higher at higher multiplication factor. In this case, even though you can change the fpd to 100MHz, but the added noise from the Multiplier may not be justified. Consider using a 100MHz reference clock.

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Hi Noel,

    Thank you for response.

     

    FM modulation was possible by setting PLL_DEN to 2 ^ 24-1.

    However, the FM deviation at 30MHz is 2.4kHz, so I want to get closer to 5kHz.

     

    I set the Prescaler to 4 and brought it closer to 5kHz, but there are some frequencies that do not lock the PLL.

    How can I lock the PLL with Prescaler = 4?

  • 0 in reply to Jun Mori
    TI__Guru** 109350 points

    Hi Mori-san,

    With fpd = 100MHz, Prescaler = 4, DEN = 2^24 and fout = 30MHz, the max. deviation we can get is 4.88kHz. It is because the FSK step is 16-bit long.

    If you need 5kHz or more deviation, will you consider to use LMX2572LP? This device has similar FSK function as the LMX2571, but its phase noise and spurs performance are better. 

  • 0 in reply to Noel Fung
    TI__Genius 12560 points

    Hi Mori-san,

    We haven't heard from you long time on this and assumed the issue got resolved. We are closing this thread and you feel free to reply on this or create new thread for further queries.

    Thanks!

    Regards,

    Ajeet Pal