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LMX2571: Using an X-tal as frequency reference

Hanspeter Oppliger
Prodigy 40 points

Part Number: LMX2571
Other Parts Discussed in Thread: SN74LVC2G14

I have noticed in the datasheet of 2015, page 11 following points to remember:

When using the Multiplier (MULT > 1), there are some points to remember:
• The Multiplier must be greater than the Pre-divider.
• Crystal mode must be disabled (XTAL_EN=0).
• Using the multiplier may add noise, especially for multiplier values greater than 6.

Question: Is it really not possible, to use the multiplier when an external X-tal is used? What is the reason, as the block diagramm doen't give enough details.

If this is true, how is then the follwing figure 60 to interpret?

I see, that with the LVPECL and the Crystal, the close in phase noise is lower. Is the main reason of the lower phase noise with the Crystal because of the missing muliplier (MULT)?

I'm looking forward to receive from you the proper explanation - many thanks in advance.

Thanks and best regards,

Hanspeter

  • 0
    TI__Mastermind 31632 points

    Hi Hanspeter, 
    1. Correct, multiplier cannot be used in XTAL input mode. 

    2. No the difference is due to slew rate mostly. A LVPECL signal is a differential signal - a square wave to be exact. This results in great slew rate even at a low OSCIN frequency such as 26 MHz that is being used for this plot. The TCXO is a clipped sine wave that has decent slew rate, not as good as the LVPECL signal but not a bad as the XTAL which is a Sine wave in this case. Sine waves suffer from low slew rate at these lower frequencies which is why we always recommend a square wave at lower frequencies when possible. 

    Regards, 

    Vicente 
     

  • 0 in reply to vicente flores prado
    Prodigy 40 points

    Dear Vicente

    Many thanks for your helpful answer. One point remains open and another cames up:

    1. What is the main reason that the phase noise of the X-tal reference is comparable to the LVPECL situation?

    2. Assuming the availability of a very fast Schmitt-Trigger gate, would I be able to improve the phase noise compared to the CS operation of the standard TCXO? I know, the uncertainties of the gate will come in as well - but do you have any experience, resp. comparisons done in the past?

    Many thanks and best regards,

    Hanspeter

  • +1 in reply to Hanspeter Oppliger
    TI__Mastermind 31632 points

    Hi Hanspeter, 
    The xtal oscillator circuit inside is of good quality in this instance. 

    2. It is possible to use a Schmitt trigger to improve the slew rate. A device like SN74LVC2G14 seems appropriate. I don't have any experience nor could find any data on this though. If you want to be certain you have good slew rate, use a square wave input. 

    Regards ,

    Vicente