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LMK03318: Crystal frequency and characteristics

Elder Costa17
Expert 2825 points
Part Number: LMK03318

I have some questions about the crystal.

1- Would a 50MHz crystal offer better jitter performance than a 25MHz one? The EVK has a 50MHz but I already have a 25MHz on the design so I would like to use the same if there is no performance degradation.

2- Related to the first, data sheet recommends 9pF load. The crystal is 18pF. Is it OK?

3- If I understand it right, performance figures in data sheet as well as Webench's simulations are based on the built-in crystal oscillator. Is it right? Is there any reason performance wise to use an external oscillator?

Thanks in advance.

  • 0
    TI__Genius 16845 points
    Hi Elder,

    A1. This depends on the oscillator performance. If the 25MHz oscillator has a phase noise profile similar to the 50MHz oscillator, then performance will also be similar.

    A2. The integrated crystal oscillator works best with crystals that have a load capacitance of 8pf to 12pf. It is also important that the crystal meets the drive level requirement of 300uW and has low ESR as specified in datasheet table 8.7. Over-driving the crystal can result in crystal reliability issues. Be aware that a mismatch in crystal load capacitance can produce a frequency error. See datasheet section 10.4.3 for more info.

    A3. Use the integrated crystal oscillator when you have a crystal that is compatible with LMK03318. Use an external crystal oscillator or an external reference clock when the crystal is incompatible, or when you want to supply an input frequency that exceeds the range of integrated crystal oscillator (10MHz to 52MHz). The reference inputs can accept clock frequencies of 1MHz - 300MHz.

    Kind regards,
    Lane
  • 0 in reply to Lane Boyd
    Expert 2825 points

    Hi, Lane.

    Re A1, I meant a crystal, using the internal oscillator. But given the A2, then it makes sense picking a 50MHz that meets the requirements.

    Thank you very much for your support.

    Elder.