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LMX2492-Q1: Maximum Loop Bandwidth

Joe Leonard
Prodigy 40 points
Part Number: LMX2492-Q1

Hello,

I am trying to determine if I can operate this component in the desired mode (FSK triggered by digital pin). Nominal Frequency 2200MHz, modulate at 10MHz, with a frequency deviation of 7MHz (0.7 Index). I would need a loop bandwidth approximately 20MHz (roughly two times the modulation rate), is this advisable with this part? On PLLatinum sim, it looks like a I can get >50 degrees phase margin, 

Kind Regards,

Joe 

  • 0
    TI__Guru* 84930 points

    Hi Joe,

    I am pretty sure that it won't work. First of all, the loop filter capacitor values are extremely small, just a couple of pF. During FSK, the PLL will be operating at fractional mode, I doubt if pF capacitors can filter the PLL noise. 

    I seldom see a practical loop filter bandwidth greater than 1MHz. People either do open-loop modulation or if possible, use a DAC instead of synthesizer.