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LMX2571: 2-level FSK modulation with high dynamic range

Hanspeter Oppliger
Prodigy 40 points
Part Number: LMX2571

Dear Ladies and Gentlemen

 

We are using the LMX2571 quite new and we like to have a 6 kSymb/s FSK modulation that is spectrum optimized and high dynamic range.

 

Do you have good suggestions of does and don't does, to have a very pure TX-signal? Currently we represent the 2-FSK levels with 8 discrete frequency shifts and approximate the low slew rate by the eight discrete steps, but maybe there is a better way to realize the modulation.

We are looking forward to your further suggestions, the application note SNAA309–May 2017 is already known to us.

So to go over I2S seems to be one solution. My question is, would it be also possible to use the PIN mode with the correct designed loop filter, allowing to have the  pulse shaping done by the loop filter?

 

I'm looking forward to your feedback, many thanks in advance.

 

Thanks and best regards,

Hanspeter

  • 0
    TI__Guru** 107810 points

    Hi Hanspeter,

    Loop filter cannot replace pulse shaping filter, they do different things.

    You have tried slowing the slew rate, if this is still not enough to reduce the modulation spectrum, you need to pulse shaping the data and I2S mode. 

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Dear Noel

    Thanks for your response:Your answer is as I expected it to be. 

    If this is the case, can you tell me, what the purpose of the three external FSK input's is. I suppose, as the LMX2571 is a very powerful device, the designers had some good ideas in mind to make the three bits accesible from external. Can you tell me what the main purpose of them is?

    I'm looking forward to your reply and remain with my best regards,

    Hanspeter

  • 0 in reply to Hanspeter Oppliger
    TI__Guru** 107810 points

    Hi Hanspeter,

    PIN mode provides a simple and straight forward way to implement discrete level FSK modulation. If the application does not have a modulation bandwidth constraint, this synthesizer allows us to do FSK modulation without complicated hardware and software.

    Remember that this device is just a synthesizer with additional features, it is not a protocol specific (e.g. Bluetooth or WiFi) ASIC.