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WEBENCH® Tools/LMK04808: How to set the VCXO freqneucy in the PLL1 of LMK04808B in WeBench Clock Architect?

Part Number: LMK04808

Tool/software: WEBENCH® Design Tools

Hi TI experts,

LMK04808B has two PLLs. The VCO of PLL1 is an external VCXO or a tunable Crystal. 

However, the entry to specify the frequency of VCXO is not found the WeBench Clock Architect design tool. Instead, the Clock Architect generate a configuration solution of LMK04808B with a specified VCO frequency of the PLL1.

If a VCXO with the frequency other than the auto-generated PLL1 VCO frequency, for example the 122.88MHz VCXO in  LMK04808BEVAL, is picked, how can I still use the WeBench Clock Architect design tool to optimize the design of two filers in LMK04808?

Best Regards!

Wallace

  • Wallace,

    Thank you for reaching out to us!

    Currently, the Clock Architect does not have an entry for specifying custom frequency of VCXO.

    This would be an area of improvement for the future.

    Regards,

    Alfred

  • Hi Alfred,

    Thank you for your answer. :-)

    LMK04808BEVAL selects the capacitor and resistor values. The filter1 for PLL1 has a  loop bandwidth of 12Hz and phase margin of 49 degree. The filter2 has a bandwidth of 424kHz  and phase margin of 76 degree..

    The WeBench Clock Architect design tool makes BW=73Hz and 69 degree for PLL1 and BW=383kHz and 72 degree for PLL2.

    However, WeBench picks a VCXO=100MHz, other than the 122.88MHz on the LMK04808BEVAL.

    Which one could be regarded as an optimal solution, WeBench, or the design of LMK04808BEVAL?

    Best Regards!

    Wallace