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LMK04828: Reference Clock

Max Pom
Intellectual 330 points
Part Number: LMK04828

Hello,

Please advise,

Is there any difference in supplying the reference clock to this device as a sinwave or a square-wave?

Can it affect any property of the device performance (for example Phase-Noise/Jitter, etc.)?

Thanks,

Max

  • 0
    TI__Mastermind 29850 points

    Hi Max,

    The big difference between sinewave/square-wave clocks is the slew rate that can be achieved. Square-wave or clipped-sine clocks tend to have better slew-rate because of the decoupling of rise/fall time from the waveshape. Sine wave slew rate is given by 2π*F*Vpk, where F = input frequency and Vpk = peak amplitude. This can cause some trouble meeting the minimum slew rate requirements at lower frequencies with sine wave clocks, while staying within the input pin limitations on minimum and maximum voltage. I've included a graph that illustrates this effect.

    The only other thing I can think of is potentially square wave clocks could couple higher order harmonics into CLKin1 through the shared power supply pin. If you're using an external VCO on CLKin1 and a reference clock at lower frequency on CLKin0, using a sine-wave clock source for CLKin0 can help minimize higher order harmonic coupling into the VCO signal (but you'll still get a spur at an offset equal to VCO ± CLKin0).

    Regards,

    Derek Payne

  • 0 in reply to Derek Payne
    Intellectual 330 points

    Hi Derek,

    Thanks for your reply.

    The datasheet (Page 12) mentions:

    "To meet the jitter performance listed in the subsequent sections of this data sheet, the minimum recommended slew rate for all input clocks is 0.5 V/ns. This is especially true for single-ended clocks"

    It also mentions (Page 97) that the input clock can be driven by a sinwave.

    From this and from your explanation I concluded, that in order to preserve the jitter/phase noise performance,

    I need to apply at least 16Vpp (28dBm) in order to keep the Slew-Rate at least 0.5V/ns (SR=2*pi*f*Vp)

    But, the maximum input power (Sinlge-Ended) is 2.4Vpp (Page 10).

    Can you explain the paradox?

    Thanks a lot!

    Max

  • +1 in reply to Max Pom
    TI__Mastermind 29850 points

    Hi Max,

    You can't meet the recommended minimum slew rate with a 10MHz sinewave, as your calculation has demonstrated. You could meet the recommended minimum slew rate with a 100MHz sine wave, since this would require merely 1.6Vpp. The LMK0482x can be driven by a sinewave input, provided the frequency of the sine wave is high enough to support the minimum slew rate within the 2.4Vpp maximum single-ended signal swing specified on page 10. I think the datasheet could do a better job of conveying the limitations of sinewave inputs at lower frequencies, so I will make a note of this for our next datasheet revision.

    Regards,

    Derek Payne

  • 0 in reply to Derek Payne
    Intellectual 330 points

    Hi Derek,

    Understood.

    Thanks a lot!

    Regards,

    Max