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LMX2571: CPFSK using LMX2571

0dBc
Prodigy 190 points

Part Number: LMX2571
Other Parts Discussed in Thread: LMX2572LP

Hi PLL team,

Is it possible to generate a continuous phase FSK signal using LMX2571 or any other synthesizer TI offers? 

Thanks.

  • 0
    TI__Genius 11900 points

    Hello 0dBc,

    We have two synthesizer products that support FSK: LMX2571, LMX2572LP

    Please refer to section "7.3.13 FSK Modulation" of the LMX2571 datasheet, and section "7.3.14 FSK Modulation" of the LMX2572LP datasheet for descriptions on what these devices are capable of.

    In both these devices FSK modulation is achieved by changing the output frequency by changing the N divider value.

    The FSK SPI FAST mode may be of interest, however because we are still changing the N-divider this would be discrete changes to frequencies.

    Thanks,

    Vibhu

  • 0 in reply to Vibhu Vanjari
    Prodigy 190 points

    Hi Vibhu,

    Is the FSK in these products "continuous phase"? CPFSK is different than normal FSK. Can I use any of the FSK SPI FAST or I2S modes in LMX2571 to generate continuous phase FSK?

    Thanks.

  • 0 in reply to 0dBc
    TI__Guru** 122250 points

    Hi There,

    we don't have any specific modulation engine inside the chip, the way that we realize FSK modulation is by changing the N-divider.

    to support bandwidth limited FSK modulation, user needs to pre-process the data before modulating the synthesizer. For example, GFSK with LMX2571/72 is possible as long as the raw data is pulse-shaped before feeding to the chip. See below appnote for the concept and implementation. I believe the same technique can be applied to CPFSK.

    www.ti.com/.../snaa309

  • 0 in reply to Noel Fung
    Prodigy 190 points

    Hi,

    Could you provide a clear answer to the following question:

    Can I utilize the LMX2571 such that the phase stays continuous when changing the N-divider while doing FSK modulation? There is no mention of phase continuity on the datasheet or the app note (SNAA309).

  • 0 in reply to 0dBc
    TI__Genius 11900 points

    Hello 0dBc,

    Unfortunately, I think the answer is no. The phase will not stay continuous when you adjust the N divider.

    Thanks,

    Vibhu