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LMK04826: LMK04826 Clkin1, Pll lock?

HENRY JUNG
Expert 2030 points
Part Number: LMK04826

Dears.

Dears.

The LMK04826 has been designed and doing test.

REF_CLK = 10Mhz.

10Mhz is supplied by Signal Generater.

When 10Mhz level is less than 5dBm, PLL lock does not work.

Even if R344 is removed and a 10Mhz level is input up to 0dBm, it becomes PLL Lock.

Why does the PLL Lock level differ?

Is 50 Ohm necessary?

How do I get a 10Mhz level to -3dBm to PLL lock?

Thank you

  • 0
    TI__Mastermind 18070 points
    Hi Henry,

    Do you mean that if input power is 0dBm, then 1) With R344, it doesn't lock 2) Without R344 it locks. ?

    Regards,
    Hao
  • 0 in reply to Hao Z
    Expert 2030 points
    Hi Hao
    without R344 lock.
    R344 is removed and it is locked with input 0dBm.
    Why does this work?
    Please let me know how we can improve.

    Thnak you.
  • 0 in reply to HENRY JUNG
    TI__Mastermind 18070 points
    Hi Henry,

    I thought I posted an additional answer but somehow it didn't show up.

    So what I said in that post was that input power or swing is not the only factor, the slew rate also matters. If you convert dBm to Vpp and then calculate the slew rate from 20% to 80% using 10MHz sine wave, you'll find that this number will probably be much smaller than the minimum input slew rate specified in the datasheet.

    A quick sanity check is to use 100MHz input instead. You may find that the device can tolerate lower input power without lock failure. So in order to use low power clock input, you need to increase the edge rate of the source.

    Regards,
    Hao
  • 0 in reply to Hao Z
    TI__Mastermind 18070 points
    As for the 50Ohm resistor. If you're familiar with transmission line theory, you can see refer to this post for more details: e2e.ti.com/.../2867177

    Regards,
    Hao
  • 0 in reply to Hao Z
    Expert 2030 points
    Hi Hao Z

    Can I remove the 50Ohm resistor?
    When the resistor is removed, the CLK input level is locked to 0dBm.
    I think it will be okay.
    Please review.

    Thank you
  • 0 in reply to HENRY JUNG
    TI__Mastermind 18070 points
    Hi Henry,

    The 50Ohm resistor is there to reduce VSWR, because the input is high impedance at low frequency. We are not sure if excessive standing wave ratio will cause any trouble, so we recommend not removing the 50Ohm if possible, although we haven't seen any problem without the 50Ohm.

    Please consider increasing the slew rate of your input instead.

    Regards,
    Hao