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LMX2571: Problem with 2FSK Frequency Deviation

Ryan.Bishop
Expert 3900 points
Part Number: LMX2571


Hello,

I'm working with one of our customers and they have the following request:

"I’m testing synthesizer LMX2571 on evaluation board LMX2571EVM. I have some problem with 2FSK frequency deviation.

  • What are relationships between frequency deviation for the 2FSK modulation scheme (the FSK modulating data pin FSK_D0 and the clock pin FSK_DV0 for FSK PIN mode are used) and registers R9 (FSK_DEV0_F1) and R10 (FSK_DEV1_F1)?
    What should be settings for R9 and R10 e.g. for symmetrical 2FSK frequency deviation +-400 kHz?

  • Also, I can’t find what LMX2571’s parameter is responsible for the modulation magnitude (scaling frequency deviation)?"

Thank you!

Regards,
Ryan B.

  • 0
    TI__Guru** 111720 points
    Hi Ryan,

    Please read datasheet Section 8.1.1 for FSK details and section 8.2.1 for an example.
  • 0 in reply to Noel Fung
    TI__Expert 3900 points

    Hi Noel,

    Thank you for directing me to the right place. I read up on those sections and I think I have a better understanding of where the customer is running into a snag.

    What they need is Fout = 915MHz, with frequency deviations of 400kHz.

    They were able to get +/- 100MHz deviations with this configuration:

    • Fosc=20,000,000 Hz
    • Fosc_div1 = 1
    • Fosc_mul = 3
    • Fosc_div2 = 1
    • DEN=8,000,000
    • Fpd = 60,000,000
    • Fvco=4,575,000,000
    • PLL_prescaler = 2
    • Fout_div1 = 5
    • Fout_div1 = 1
    • Fout = 915,000,000

    However, with the above configuration even the maximum number of frequency steps in registers R9, R10 can't get the deviation to 400MHz (FSK_DEV0_F1=32765 and FSK_DEV1_F1=32768). In an attempt to get +/-400MHz deviations, they modified the configuration to this:

    • Fosc=20,000,000 Hz
    • Fosc_div1 = 1
    • Fosc_mul = 6
    • Fosc_div2 = 1
    • DEN=4,000,000
    • Fpd = 120,000,000
    • Fvco=4,575,000,000
    • PLL_prescaler = 2
    • Fout_div1 = 5
    • Fout_div2 = 1
    • Fout = 915,000,000

    Those settings got them close but not quite (388MHz).

    Can you help suggest the optimal settings for the chip, when LMX2571EVM Board is used (Fosc=20 MHz) for the requirements: Fout = 902-928 MHz, data rate 1.1Mbps, frequency deviation 400 kHz?

    I hope all that was clear. Thank you very much!

    Regards,
    Ryan B.

  • 0 in reply to Ryan.Bishop
    TI__Guru** 111720 points
    Hi Ryan,

    400kHz deviation is a big number, I think it is still doable at 9xxMHz output, please verify it in the EVM.
    In order to get big deviation, according to Equation 4 in the datasheet, we have to decrease DEN and/or increase fpd.
    EVM has a 20MHz XO, I suggest put MULT = 4. This gives fpd=80MHz, which is an optimum fpd frequency.
    DEN could be 80000. In the datasheet, I said DEN should be equal to 2^24 whenever FSK is used. This is required so as to ensure FSK works even in an integer channel (fractional NUM = 0). If the application does not include integer channel, DEN could be some other values.
    For example, in this case, if we put fpd=80MHz; DEN=80000, you should be able to get +/-400kHz deviation between 902 to 927MHz (I assumed channel spacing is 1MHz). At 928MHz, this is an integer channel, you can get positive deviation but not negative deviation. At this channel, we can change MULT to 5 to make it a fractional channel.