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Setting LMX2531 internal loop filter components in Clock Design Tool software

PJB
Prodigy 30 points

Does anyone know how to select the advanced loop filter component selection in Clock Design Tool?

I'm trying to set the internal C and R loop filter values by choosing one of the`drop down box settings in the advance section of the loop filter menu. Looking at the phase noise results they don't seem to have any effect, which strikes me as wrong / odd.

Any ideas on why this doesn't appear to be working, or what I've done wrong, would be gratefully received.

PJB 

  • 0
    TI__Mastermind 28031 points

    PJB,

    Let's suppose you have the phase noise open in one box and the loop filter design box open in the other one.  If you select different components from the drop down list and do nothing else, it will NOT change the phase noise, because these are interpreted as design constraints, not the final design.   If you press the "calculate" button, you might see some small changes in the phase noise but they will be relatively small because the tool tries to re-design the fliter for the same bandwidth and phase margin after accounting for these extra components.

    If what you want to do is start with a fixed loop filter on your board and see the impact of switching in different R&C values for the internal values, the way to do this is not the internal loop filter calculator, but rather to go to the LOOPFILTER box (make sure 4th order is selected), and type the values that you want.  You can get the allowable values from the datasheet or from the drop down box in the loop filter calculator.

     

    Regards,

    Dean