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LMK04832: Relationship between channel spacing and phase detector frequency

Ee Wen Quek
Intellectual 300 points
Part Number: LMK04832
Other Parts Discussed in Thread: LMK03318

Hi,

Is there any relationship between channel spacing and phase detector frequency, fpd? Does fpd must equal to channel spacing or a factor of channel spacing? I am using LMK04832 to design an output clock frequency with a range of 100MHz to 800MHz with 10kHz resolution using dual loop design. Concern on PLL 2 only where the input clock reference is 100MHz, I noticed 10kHz resolution is not achievable I just change the divider at output side ONLY. However if I changed VCO range, N and R values will need to change as well, at the same time fpd will change. Thus, I just wondering whether the fpd always equal to channel spacing or they are independent?

Thanks.

  • 0
    TI__Guru** 107230 points
    Hi Quek,
    For an integer-N PLL, phase detector frequency cannot be greater than the channel spacing, in general, they are equal to each other.
    LMK04832 is a dual integer-N PLL, in most of the applications, the VCO frequency is fixed and never change during normal operation.
    Even if you can change the fpd of PLL2 to 10kHz, the phase noise is going to be bad because the N-divider becomes very big.
    If your application requires sweeping frequencies, a fractional-N PLL is needed.
    You may consider our synthesizer products, part number begins with LMX.
    You may also consider LMK03318/28 clock generators, they have fractional-N PLL inside.