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LMX2820: How do differential sinewave OSCIN inputs affect phase noise of LMX2820?

ibrahim akcali
Prodigy 20 points
Part Number: LMX2820

In our design, we think benefitting from a low phase noise 100MHz sine wave oscillator regarding of carrying signals over long distance. Considering isolation issues, we thought carrying the signals as differential pairs. Can you inform us about the solutions comparing optimum drive level, differential pair or single ended routing of sine wave signal for best phase noise performance of LMX2820?  On the other hand, we don't want to degrade phase noise regarding slew rate limitations. Should we use CMOS rather than sine? How much does the phase noise performance differ between two options?     

  • 0
    TI__Guru** 107960 points

    Hi Ibrahim,

    I don't see a noticeable difference in term of phase noise between using 100MHz sine wave and square wave. If you have to carry the 100MHz signal for a long distance, a good quality coaxial cable is better than a twisted pair wire. Both of these cables are low pass filter, the high frequency content of a square wave signal will eventually get filtered out, the signal at the output end will not be a square wave anymore. I suggest sine wave with good quality coaxial cable.

  • 0
    Prodigy 20 points

    Hello Noel, 

    We carry the clock signal as sine wave to the pcb. Then, we translate to differential signals with balun in the pcb due to isolation problems and we carry as differential pair on pcb to the synthesizer. We have seen the difference on the drivel level in between single-ended (15dBm) and differential signals (7dBm). Can you inform us about the optimum drive level for phase noise? If we need higher drive levels, do you suggest us to use only single ended routing for OSC_IN?     

  • +1 in reply to ibrahim akcali
    TI__Guru** 107960 points

    Hi Ibrahim,

    For single-ended input, I would recommend close to 2Vpp. 

    For differential input, close to 1Vpp on each input pin.

    How long is your differential trace? Sounds to me that you have a very long diff trace surrounded by other jammers. 

  • 0
    Prodigy 20 points

    Hello Noel, 

    Thank you for your quick answers and drive level. The length of differential signal is less than 10cm. 

  • 0 in reply to ibrahim akcali
    TI__Guru** 107960 points

    Hi Ibrahim,

    10cm is not very long, it should be fine.