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LMK04826BEVM: LMK04826B Evaluation Board with different external VCO

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Part Number: LMK04826BEVM

Hi to everyone!

We have recently aquired a LMK04826B Evaluation Board. This borad has an external VCXO (CHVD-950) with a nominal frequency of 122.88MHz. We have unmounted this VCXO to mount an OXCO (tfc-OS240) with a nominal frequency of 10MHz.

The new OCXO has 5 ports, Vcc, Vtune, Out and GND have been directly connected to the points where the previous VCXO was connected. The fifth port is an output (Vref) that has been left open.   

The point is that we are unable to achieve locking in the PLL1 and we do not know why. It is possible to make this change of VCXO? Have we forgotten something?

Thanks in advance,

Juan.

  • Hi Juan,
    LMK0482x family is dual PLL jitter cleaner, when PLLs locked, frequency offset ppm value would be the same for CLKin, OSCin, and CLKout.
    For example, CLKin is a +/- 50 ppm XO, then VCXO also would locked on +/-50 ppm, final CLKout also is +/- 50ppm on target frequency.
    The VCXO, as OSCin input and PLL1 VCO position, normally has several hundreds ppm tuning range, but OCXO only has a very small tuning range, tfc-OS240 is +/- 1ppm.
    When your CLKin for LMK04826 has a frequency offset larger than +/- 1ppm, OCXO has no way to reach target frequency, then PLL1 always unlocked.
    OCXO is not proper to replace VCXO in dual pll mode, but if OCXO tuning pin has other control method other than LMK0482x PLL1, then LMK0482x PLL2 can work with a OCXO input on OSCin pins.

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