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LMX2541: Loop filter design tool for external VCXO instead of internal VCO

Part Number: LMX2541

Hi Clocking team,

I have a customer that would like to know if there is a tip/trick to simulate the external VCXO instead of the internal VCO when doing the loop filter design in the Clock Design Tool (

Is there some additional information / steps needed to do the loop filter for an external VCXO with the internal VCO bypassed?

Should PLLatinum Sim be used for this loop filter design instead of the Clock Design Tool?

I know PLLatinum Sim specifically shows options for selecting external VCO when selecting the LMX2541xxxx devices.
Would this be the correct method to design the loop filter for use with an external VCXO?  Or is Clock Design Tool suitable?



  • Hello Ben,

    We no longer use the CLOCKDESIGNTOOL for simulations, PLLATINUMSIM-SW is the recommended tool to use.

    When using the external VCO/VCXO to get an accurate simulation you must import the phase noise metrics of the VCO/VCXO. You can also do this to import a custom OSC source.

    To do this select Feature Level = Intermediate or Advanced.

    Go to the Phase Noise tab and on the right side of the tool you will see options of "Load Data" for both an input source such as the OSC input and VCO noise source.

    Click on the radio button labelled Load Data.

    You can then import a .txt file with the VCO/VCXO source phase noise metrics.

    If you click on the beside the OSC, PLL and VCO Noise heading in the top right corner, a pop-up will appear describing the format needed for the .txt file.

    The format should be offset (Hz), Phase Noise (dBc/Hz) + <Carriage Return>. To separate the offset and phase noise, you can use comma, space or tab.