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LMK04828 question please

John Reyland62
Intellectual 375 points
Other Parts Discussed in Thread: LMK04828

Hello Clock Forum Master,

I have an ADS54J60 Eval card that has an LMK04828 on it.    Looking at the Clkout0 (pins 1) on a scope, I notice that the sine wave gets more distorted as I increase the Dclk divider.    My Dclk0 outputs ( pins 1,2) are not terminated at all, just a high speed scope probe on pin 1.   Is there some kind of a "sweet spot" for these dividers?

Have seen anything like this please?   

Thanks

John Reyland

  • 0
    TI__Mastermind 28560 points
    There is no sweet spot for the divider.

    I'm not familiar with the PCB so I'm not sure how well the device clock traces are optimized for maximum signal integrity. So as you sweep over frequency you may begin to see impacts of reflections at some frequencies. A different board may have same issues at different frequencies. You mention probing DCLKout0 pins and they are not terminated, in this case I wouldn't be surprised if you see distortion because of reflections. Ideally if things are terminated well there should be little to no impact. You may be able to measure Vpp at different points along the trace and determine the VSWR by the ratio of max Vpp to min Vpp. From VSWR you can determine return loss which is tell you how good (or bad) the matching is.

    73,
    Timothy
  • 0 in reply to Timothy T
    Intellectual 375 points
    Hi Tim,

    That makes sense, I should have thought of reflections. Did you see my other question about how sometimes one of the LMK04828 clock outputs goes to zero after outputting correctly for about a second. Is there some kind of an protective shutdown?

    Thanks (what is 73, some kind of TI code number?)
    John Reyland
  • 0 in reply to John Reyland62
    TI__Mastermind 28560 points
    73 is amateur radio jargon for best regards. So it serves as a signature, calling card, and conversation piece.

    73,
    Timothy