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LMX2615-SP: SYSREF and SYNC

Azad Jalali
Prodigy 91 points
Part Number: LMX2615-SP
Other Parts Discussed in Thread: TIDA-01024, LMK04832

This is probably trivial, but I didn't see it explicitly stated in the datasheet. If we are using the SYNC functionality to synchronize two LMX2615-SP ICs in a JESD204B application, will the SYSREF signals generated by each in Master mode (as opposed to Repeater mode) also be synchronized? If so, I have some follow-up questions:

  1. Is there any performance benefit to using Repeater mode over Master mode? We have an LMK04832 feeding several LMX2615s, similar to what is shown in TIDA-01024. There, it seems like the LMK04832 is still generating SYSREF (see first pic), and I couldn't figure out why or if we should do the same. I'd think it would be more efficient to have the LMX2615s each produce their own SYSREF, since they're already synchronized by SYNC.
  2. This might be tougher to answer, but can we expect that SYSREF will experience similar variation to the device clock, as outlined in section 7.3.14? I'm hoping that the variation is in the VCO and that they mostly move together.  (see second pic)

  • +1
    TI__Guru* 81580 points

    Hi Azad,

    SYSREF clock is synchronized with its RFout clock, so SYSREF clocks from multiple devices will be synchronized too. Similar phase variation as the RFout clock is expected as their source is identical. 

    The SYSREF feature of LMX2615 is intended to solve the problem that there is no clock device in the application system that is able to generate SYSREF clock together with DCLK. LMK04832 is a powerful device that support both clocks. So it is up to your decision to fully unitize the LMK04832 or not. I would say, PCB layout would be one of the factors we should consider to select which device to generate the SYSREF clock. LMX2615 is able to generate SYSREF clock, so maybe it is a good idea to have it generated SYSREF clock as well as RF clock. 

  • 0 in reply to Noel Fung
    Prodigy 91 points

    Noel, okay that makes sense. We're trying to squeeze as many clocks as we can out of the LMK04832 so in this case it is attractive to have the LMX2615s generate SYSREF. The finer analog delay is also nice to have.

    One more question about tradeoffs, can you confirm that the SYSREF analog delay is available in both repeater and master mode? I believe this is substantiated in Figure 27 but I would like to double check.

    Thanks,

    Azad

  • 0 in reply to Azad Jalali
    TI__Guru* 81580 points

    Hi Azad,

    Yes, it should be, I will check this out tomorrow.

  • 0 in reply to Noel Fung
    TI__Guru* 81580 points

    Hi Azad,

    I just checked, yes, the analog delay applies to both repeater and master mode. 

  • 0 in reply to Noel Fung
    Prodigy 91 points

    Great, thank you!