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LMX2572: SYNC functionality

Daniel Stracke1
Prodigy 90 points
Part Number: LMX2572

Hi,

I'm wondering about the exact function of the SYNC pin.

At the moment I'm trying to sync two LMX2572 with different output frequencies. These frequencies have a specific distance and I need them to be synchronised in a way, that they meet at "Zero-Phase" every n cycles (see the attached picture as an example).
Can this behavior be achieved using the sync pin, or can I just achieve it with the mesh seed functionality?

Yours,

Daniel

  • 0
    TI__Mastermind 18070 points

    Hi Daniel,

    SYNC function makes sure that the delay from input to output is deterministic. In other words, the phase relationship between input and output does not change after power cycle or re-calibration. SYNC by itself does not provide zero delay.

    After obtaining deterministic phase relationship with SYNC, you can then tune the absolute phase difference using MESH_SEED.

    Regards,

    Hao

  • 0 in reply to Hao Z
    Prodigy 90 points

    Hi Hao,

    thanks for the answer, but my issue is a little different:

    to be specific, I want to create for example a frequency of 2 GHz with my first PLL and with the other I want to create a frequency of 2.0125 GHz.
    Now I want them to be "kind of" in phase, which means I want them to do simultaneous zero crossing every 80 nanoseconds (1 / 12,5 MHz, since the freqencies have a distance of 12.5 MHz), similar to the picture I send before. If they are not "in phase" they will never do zero crossing at the same time instant.

    For this I was asking, whether it is possible to achieve this behaviour using the sync pin. And if so, which values of the PLLs should be set in which manner?

    My reference frequency is 100 MHz at the moment, and I do not change them, which means the Phase-detector frequency is 100 MHz too.

    Regards,

    Daniel

  • 0 in reply to Daniel Stracke1
    TI__Mastermind 18070 points

    Daniel,

    Theoretically this is possible. Like I said previously, you can use SYNC to get deterministic delay and use MESH_SEED to compensate for phase misalignment. Since MESH_SEED has very fine step size, you can achieve pretty accurate zero crossings. However, it may be very difficult to implement as you have to manually tune the MESH_SEED to make them in phase.

    Regards,
    Hao

  • 0 in reply to Hao Z
    Prodigy 90 points

    Hi Hao,

    thanks for this explanation. I think I got the point now.

    Regards,

    Daniel