LMK04110: Regarding frequency of VCXO

Hello Pearl,

My understanding is that you are...

• using the LMK04110 as a jitter cleaner.
• You have a 100 MHz input clock and what a cleaned version, so you output at 100 MHz also.
• Your 100 MHz input could vary by +/-5600 ppm.

You have a few options for VCXO frequency.  Since you are using a clocking device with an integer-N PLL, you want to pick a VCXO frequency that has a good integer relationship to the VCO frequency of your clocking device.  Since you will use 1200 MHz for the VCO frequency, I suggest...

• You could use a VCXO 100 MHz / n; where n >= 1.
  • So for example 50 or 10 MHz VCXO could also work.
  • The higher the VCXO frequency, the better the performance of the output.
• If you used a 100 MHz VCXO, you could use a single loop device which allows for use of an external VCO (or VCXO).
• Depending on your jitter requirements, you could use a single loop device, the VCXO does very good jitter cleaning at low offsets.

The problem you will run into, is the tuning range of the VCXO however.  +/- 5600 ppm is quite large!  Many will tune a smaller range like +/- 100 ppm.  You many need to look into an external VCO which would not have as good jitter cleaning, and you may be better of with a single loop clock generator.  Then you don't have to have an external VCXO + clocking chip.

73,
Timothy