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LMX2581: Reference Oscillator Input Levels

Charles Luke
Intellectual 270 points
Part Number: LMX2581

Data sheet on Page 7 says OSCIn input voltage min=0.4Vpp, max=1.7Vpp.

Figure 12 and 13 show a much lower sensitivity, on the order of -27dBm.

Can you explain the differences in the data sheet?

Also, I'm using a CMOS TCXO and it appears the Hi-Z of the CMOS output is making this Ref OscIn input susceptible to interference from the PLL output. Every time I get close to a harmonic of 38.4M, I'm seeing spurs equal to the desired signal output. On the eval card, if I drive this Ref OscIn with an external 50 ohm Signal Gen, I don't get the spurs. I am using the LMX from 740 to 2500MHz.

I think I need a voltage divider to lower the termination on the Ref In but I need to know how low I can drop this level. Also, any suggestions for interfacing a CMOS output ref osc to the LMX2581 Ref Input?

  • 0
    TI__Guru* 88960 points

    Hi Charles,

    If your TCXO output is 3.3V square wave, try below circuit. You may need to adjust R14 to compromise spurs and phase noise.

  • 0 in reply to Noel Fung
    Intellectual 270 points

    Data sheet on Page 7 says OSCIn input voltage min=0.4Vpp, max=1.7Vpp.

    Figure 12 and 13 show a much lower sensitivity, on the order of -27dBm.

    Can you explain the differences in the data sheet? Please answer this.

    TCXO output is 0.8Vpp. part number is 

    TG2520SMN 38.4000M-MCGNNM3

    from Seiko Epson. The test circuit shows a load of 10K || 10pF with a DC block of 0.01uF. Not sure it will drive 100 ohms like you show.

  • 0 in reply to Charles Luke
    TI__Genius 12280 points

    Hi Charles,

    Noel is out of the office tomorrow so he will get back to you on Thursday.

    Best,

    Evan Su

  • 0 in reply to Evan Su
    TI__Guru* 88960 points

    Hi Charles,

    0.4V to 1.7V ensures that the PLL noise performance meets datasheet specification. PLL can still operate at -27dBm input, however, noise performance will degrade. 

    Your TCXO output is clipped sine wave, not CMOS. This type of output cannot drive small load such as 50Ω or 100Ω.

    You can simply AC-couple TCXO output to OSCin.

    If the spur you are seeing is due to crosstalk from OSCin, then it is true that the spurs will depend on the OSCin signal and interface. 

    Some peoples will add a filter after TCXO to make the signal more like a sine wave. This approach may reduce spurs but may hurt PLL noise as the slew rate of the input clock becomes worst.