Clock Generator with hitless switch-over. AD9557, AD9548, HMC7044

Hello,  Can you help recommend the best part?  

I'm looking for a Clock Generator that can support 128MHz hitless switch-over in a non-SONET application.

The datasheets on AD9557, AD9548, HMC7044, and other parts  do not have much detail on hitless switching.  

How closely would the REFA and REFB need to be? 

Is there an ADI tutorial on hit-less switching?

  • 0
    •  Analog Employees 
    on Oct 31, 2016 4:13 PM


    I just want to start with clarifying the term "hitless" switching as there is some ambiguity in its use in industry. At Analog Devices, hitless switching was defined as the process of switching from a primary line card reference to a secondary line card reference without losing framing synchronization, while limiting the frequency deviation at the output clock resulting from the switch. To maintain frame synchronization, steady state phase alignment between input and output clocks must be preserved through the switching event. From the AD9548 datasheet, 'Hitless Reference Switching (Phase Slew Control)' section:

    Hitless reference switching is the term given to a reference switchover that limits the rate of change of the phase of the output clock while the PLL is in the process of acquiring phase lock. This prevents the output frequency offset from becoming excessive. 

    I distinguish because many of our devices have a reference switching mode that does not phase align the input and output clock, which truly allows for minimizing the frequency disturbance at the output clock associated with the switching event. We call this Phase Build-Out switching. From the AD9548 datasheet, 'Phase Build-Out Reference Switching' section:

    Phase build-out reference switching is the term given to a reference switchover that completely masks any phase difference between the previous reference and the new reference. That is, there is virtually no phase change detectable at the output when a phase build-out switchover occurs.

    Currently, only the AD9548 and HMC7044 support hitless reference switching, while the AD9548, AD9557, AD9558, AD9559, and AD9554 support phase build-out reference switching.

    Neither phase build-out or hitless reference switching impose any requirement of phase alignment between the primary and redundant reference for proper operation. However, the duration of the switching transient of a hitless switch will be affected by the initial phase offset present between the two references, as the output must ultimately align to the new phase alignment.

    There is an Application Note describing Phase Build-Out and Hitless reference switching and the transients associated with each in numerous primary/redundamnt clock relationship scenarios making its way through the publishing process, but I do not believe it is not currently publicly posted. Check back for a link to the App Note when it becomes available.

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:37 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin