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LMK05318B: The allowed frequencies for the XO input.

Seung-Gon Hong
Prodigy 160 points
Part Number: LMK05318B

Hi,

I am reviewing LMK05318B for my design. I tried to put the 10MHz OCXO clock into the XO input and to set it to 10MHz using the TICS pro wizard as shown below, but only a specific frequency is recommended for the following instructions.

However, I also discovered that EVM made a footprint to use 10MHz TCXO as shown below. Although it is "DNI:...

Question 1. Can I use 10MHz for XO clock? If yes, what are the downsides compared to using the recommended clock?
Question 2. Does the ppm(Frequency stability) of the output clock depend on the crystal (or TCXO) connected to the XO?

I tested with EVM to check phase noise and stability of LKM05318B. When comparing OCXO 10MHz and crystal oscillator on EVM for XO clock, crystal oscillator was better for phase noise, but OCXO 10MHz was better for stability. Please give me some advice on how I can choose the best Stability and Phase Noise.

Thanks.

DNI지만..
  • 0
    TI__Expert 8585 points

    Hello,

    1. A 10 MHz XO clock can be used when the DPLL is disabled. When the DPLL is disabled, you can no longer lock to a reference input and you will lose the output frequency accuracy that comes with using the DPLL. This is the downside of using an XO frequency that is an integer multiple of the VCO frequency (2500 MHz/10 MHz = 250 which is an integer).

    2. When the DPLL is not used, the frequency stability of the output clock will depend on the ppm accuracy of the XO/TCXO connected to the XO input. When the DPLL is used, the frequency stability of the output clock will depend on the ppm accuracy of the reference input.

    My recommendation would be to use a high frequency XO (38.88 MHz or 48.0048 MHz) and provide a reference input to PRIREF (25 MHz is a good option). The high frequency XO will result in great output phase noise performance and the reference input will result in great frequency stability (due to the DPLL locking to the reference input).

    Regards,

    Kia Rahbar