LMK04002B EVAL ERROR in input frequency

Hello Fernando,

In real life you should be able to get LMK04002B to lock to any frequency as long as it is within specification.  My guess is that you are using a signal generator to provide a clock input reference and that the slew rate presented to the CLKin input is too low as a minimum of 0.15 V/ns is specified for the CLKin.

Note that a 0 dBm 12.288 MHz sine wave has a 20 to 80% slew rate of 0.023 V/ns and a 0 dBm 122.88 MHz sine wave has a 20 to 80% slew rate of 0.23 V/ns slew rate.  Please confirm the slew rate you are providing to the LMK04002BEVAL clock input.  It is possible to lock the LMK04002B device with less than 0.15 V/ns... increasing the power of a 12.288 MHz signal from a signal generator may do the trick.

 > Please advise if your still having difficulty locking PLL1 and slew rate is in the specified range.

Also, for intended performance, if the phase detector frequency is changed - the loop filter should be updated for intended performance.  I.e. the evaluation board is designed to operate with an PLL1 N = 120, meaning the phase detector frequency should be 1.024 MHz no matter the input frequency.  As such the PLL1 R for a 12.288 MHz signal should be 12.  This will result in the same loop filter dynamics for PLL1.

  - The NCDT will design for the largest possible phase detector frequency because it is a simple matter to re-program the PLL1 R for a lower phase detector frequency, while sometimes it may not be obvious what the maximum PDF will be.

  - Also, high PDF frequencies allow better PLL performance.  This is especially important for PLL2... but not as critical for PLL1 because typically the PLL1 loop bandwidth is designed to be narrow to attenuate input jitter.  A narrow loop bandwidth will minimize the phase noise contribution from the PLL and allow the external VCXO or crystal noise to dominate.

Final note, loading the actual CLKin reference and VCXO phase noise profiles into NCDT allows the user to re-calculate the PLL loop filters using real world phase noise references for best integrated jitter performance, and allows the tool to provide the most accurate phase noise simulations.

73,

Timothy