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LMX2581: Can we analog Frequency Modulate on LMX2581

Simon Brown
Prodigy 20 points
Part Number: LMX2581

We are trying to implement FM on a 450MHz output signal. which we get an response ,( AM implement on loop filter)

but this is not stable or consistant over our Audio frequency range ( 1-40KHz)

Do you think this is possible to get reliable frequency deviation with this method?

  • +1
    TI__Guru* 87210 points

    Hi Simon,

    If you want to do FM on LMX2581, you could modulate the reference clock of LMX2581. Use a VCTCXO or VCXO as the reference clock of LMX2581. PLL loop filter has to be greater than 40kHz because your audio frequency is up to this frequency. You may also need to talk to the XO vendor to tailor-made a good frequency response VCTCXO or VCXO. 

    You could also modulate the VCO of LMX2581 with an active loop filter. Since the Kvco of the VCOs is quite large, the required audio signal swing may be very small. PLL loop bandwidth has to be smaller than the lowest audio frequency. I am not sure if your lowest audio frequency is 1Hz or 1kHz, but loop bandwidth of 1 Hz is not practical. 

  • +1 in reply to Noel Fung
    Prodigy 20 points

    Thank you, we will investigate both these options