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LMK04828: Matching a single ended TCXO to the input of a LMK04828

Gregory Duperon
Expert 6540 points
Part Number: LMK04828
Other Parts Discussed in Thread: LMK04808

I have a customer designing with LMK04828.

They are using the Crystek CVHD (at a different frequency).  Here are their questions:

 

 
Presumably, when XTAL is used, C4 would be DNP C4? 
R10, R16, C8, C36 would be fitted to connect the XTAL to OSCin_P?  Some of these capacitors are used to match the input to the VCXO?
 
The CVHD has a load capacitance of 14pF, whilst I believe the LMK presents a 1pF input.  I might be misreading the datasheet as the section is as follows:
 
 
I know that the OSCin for PLL2 supports frequencies up to 500MHz.
 
Can you clarify input capacitance of the OSCin port?
 
Any guidance for presenting the correct load capacitance for the VCXO?
  • 0
    TI__Mastermind 28550 points

    The LMK04828 supports placing a crystal on OSCin.  However if you're using a CVHD VCXO, you are using a part with an output buffer so there is no need to worry about the crystal specifications.  The schematic as given above can be used except you may pick different C4 or C33 values (or just remove C33).  C33 can help with filtering some higher order harmonics of the VCXO.
    > So in this case, the 10 to 40 MHz is not applicable, the up to 500 MHz is what you'll be working with.

    If you do want to use the crystal feature, the LMK04808 datasheet has some more application info/example in section 9.1.8 on page 109 you may look into.

    Gregory Duperon said:
    The CVHD has a load capacitance of 14pF, whilst I believe the LMK presents a 1pF input.  I might be misreading the datasheet as the section is as follows:


    Without confirming the CVHD datasheet, that load capacitance would be what the output of the CVHD is measured at, so our device would provide much less of a load, however you should consider all traces, etc.  In the event your final capacitance was greater, it simply means the edges may be rolled of a bit more or that your input amplitude may be slightly less.  So no significant concerns.

    Let me know if this answer misses the mark or helps clarifies your question.

    73,
    Timothy