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LMX2581EVM OscIn Power Levels

Other Parts Discussed in Thread: LMX2581EVM

I am evaluating the LMX2581EVM and I was wondering if I could clock the device with an external 10 MHz Wenzel oscillator spec'd for +13 dBm output. My real question is, is there a specific power level or power range that OSCIn should be in order to characterize the LMX2581EVM? Does it depend on frequency? I could not find any information in the user's guide so any help is much appreciated. Thank you.

  • Hello,

    In genreal, the part likes a fast slew rate.  For the best phase noise, you want the highest output power level.  However, we do spec a max level of 1.7 V, which I think is about +8 dBm, so you might want to reduce this down to +8 dBm.  

     

    Regards,

    Dean

  • Thank you for finding that spec. I did some more research and I read the following in the User's Guide. I think the resistor modification matches the OSCIn input to 50 Ohms, which means that I will indeed need to make sure that the max Vpp stays below 1.7V.


    Now that I look at it closer, is there a typo? It tells me to "change R15 and L3 to 0" but then "make R15 51 Ohms". Since we are trying to make the impedance looking out from the OSCIn port to 50 Ohms, shouldn't R16 be 51 Ohms instead?

  • Hello,
    For the L3, definitely make this 0 ohm. For an external OSCin, you would disconnect R14, make R15 51 ohm, and R16 and R13 equal 0 ohm. Shorting R15 to ground as the instructions say would kill the input signal. In your case, maybe you might want to make a pad like R13=18 ohm, R15=68 ohm, and R16 = 18 ohm.
    Regards,Dean
  • Thank you for the help!