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LMX2492EVM: Phase noise characteristic when denominator=2^24

AY
Prodigy 180 points
Part Number: LMX2492EVM
Other Parts Discussed in Thread: LMX2492

As per the datasheet of LMX2492, the denominator of the fractional divider is forced to 2^24 when the chip is running the frequency ramping. We are investigating the phase noise at a constant frequency 10.525GHz to begin with (see the graph attached).  

1. a fractional of 10485760/16777216, strange phase noise behaviour is observed .

2. a fractional of 5/8, the phase noise seems to be matching the theoretical calculation

Question:

1. Why is there a big difference in the phase noise characteristic when a fractional circuit with the max denominator is used?

2. Is there any difference in setting of the denominator to 2^24 in CW mode and in ramping mode? 

  • 0
    TI__Mastermind 27581 points
    AY,

    The fraction of 5/8 and 10485760/16777216 are the exact same and should produce the exact same spectrum UNLESS the initial state of the modulator is not 0. I think that you will the same spectrum if you set FRAC_ORDER="Reset Modulator" and then change it to the desired order.

    However, sometimes you can get a non-zero initial state unintentionally. For instance, if you toggle the modulator state from 2nd to 3rd order, or maybe if you power the device down and up with the powerdown bit.

    I think you are seeing the effect of this nonzero state.

    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 180 points

    I will have to check this setting when I am back in the lab again to see if this solve the problem.

    If we are running in the ramping mode, do we need to reset the modulator too as part of the sequence?

  • 0 in reply to AY
    TI__Mastermind 27581 points
    AY,

    In ramping mode, the reset is unnecessary as the fractional numerator is rolling anyways.

    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 180 points

    Hi Dean,

    We couldn't find the FRAC_ORDER="Reset Modulator" in the TICS Pro program. There are only options for Integer mode, 1st, 2nd, 3rd, and 4th order modulation and reserved as options. How do we reset the modulator?

    Which register do you mean?

  • 0 in reply to AY
    TI__Mastermind 27581 points

    AY,

    If you do "Integer Mode", it should be the same as resetting the modulator.

    Regards,
    Dean