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LMK04828: VCO leaks out of VCC_VCO, CPOUT2

Part Number: LMK04828
Other Parts Discussed in Thread: LMK04826, , LMK04832

We are trying to generate 800 MHz clock using a 120 MHz reference for PLL2.  We use VCO0 at 2.4 GHz. 

We observe the 2.4 GHz leaking to the power pin(VCC_VCO) at -38 dBm and at the CPOUT2 at -45 dBm. ( I have used 10 pFs on the bypass line caps)

However, if we  switch to VCO1 ( changed the reference to 100 MHz)  at 3 GHz, the level of leakage at the power pin and the CPOUT2 is -60 dBm.

Could you please explain? Is there a way to reduce the leakage? 

  • Hello,

    These are two different types of VCOs, based on the design it makes sense that there is leakage from the VCO0 core more than the VCO1 core to the other pins.

    In the LMK04826 VCO1 is ~2.5 GHz and will have the lower leakage amount. Unfortunately it will not tune down the 2.4 GHz. I've also seen the higher crosstalk onto SDCLKout1. However when signals are taken differentially there is a good reduction in crosstalk.

    Normally leakage of the VCO frequency is not an issue because it is already a harmonic of the other output frequencies. Is your design having some sensitivity? Another possibility is the LMK04832. It is pin compatible with LMK04828, it has a VCO1 which operates to 3200 MHz which would also enable generation of 800 MHz.

    73,
    Timothy