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LMX2572: Evaluation of phase lock

Masakazu Adachi
Intellectual 2500 points

Part Number: LMX2572

Hello team,

My customer found that the LMX2572, when combined two LMX2572 each phase changes 180 degrees(Not synchronized). 

Details:

1. Circuit configuration and register parameter as follows.

2. At this time, hitting the trigger multiple times will change the phase relationship between each other by 180 degrees.

3. Changing the parameter from R=2, PreR=1 to R=1, PreR=2 solves this problem.

Questions:

1. Is there any difference in the synchronization sequence between R and PreR counters?

Regards,

Adachi

  • 0
    TI__Mastermind 35170 points

    Hello Adachi-san,

    I'm not sure. Looking at the SYNC categories in Figure 170 of the LMX2572 datasheet, this would be a Category 3 SYNC because FOUT % FOSCin != 0. So the SYNC timing is critical. Are you certain that the SYNC is always being delivered within a valid setup and hold window for the input clock? Ideally the SYNC should be delivered on the falling edge of the OSCin clock, though with 6MHz you could easily deliver the SYNC at 90° or 270° instead of 180° offset to OSCin edge and still meet setup/hold. It may be the case that the SYNC signal is retimed differently depending on whether the PreR or R divider is used, i.e. perhaps by using PreR=2 you are effectively reducing FOSCin to 3MHz for the purposes of the synchronization. That would put you in Category 1a SYNC, in which case VCO_PHASE_SYNC_EN could be disabled and the devices should still be in phase without any SYNC pin toggle. I recommend trying an experiment where VCO_PHASE_SYNC_EN=0, PreR=2, and POR the two devices several times; if I am correct, they should remain synchronized with no 180° phase offset at all times. Please let me know if this works. It would be helpful for updating our guidance on synchronization categories in the datasheet.

    Regards,

    Derek Payne