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LMX2594: N Divider Restrictions in SYNC Mode

Tom Wallace
Prodigy 10 points
Part Number: LMX2594

When using SYNC mode, the N-divider and the fractional dividers are recomputed to account for the effects of the channel divider (IncludedDivide).

Are the minimum N-divider restrictions listed in Table 2 applicable after the N-divider has been divided by IncludedDivide, or do they only apply to the value of N before correction for IncludedDivide?

  • 0
    TI__Mastermind 33760 points

    Hi Tom,

    The restrictions in Table 2 are after the IncludedDivide. If your IncludedDivide is 4, and your N is 28, your total divide is 112 and the restriction applies to the 28 part only.

    Regards,

    Derek Payne

  • 0 in reply to Derek Payne
    Prodigy 10 points

    If that is correct, and I would be happy to have confirmation from someone at TI, then that makes some frequencies inaccessible for operation in SYNC mode, because some N's will become too small after adjustment for IncludedDivide.

    For example, in non-SYNC mode, 5038 MHz can be generated from an 80 MHz reference with N = 125, num = 19, den = 20, and chdiv = 2.

    In SYNC mode, IncludedDivide is 4, and after adjustment for that, N = 31, num = 39, and den = 80. Since f_osc = 10.076 GHz, the minimum N from Table 2 is 32, for a MASH order of 1. An N of 31 can only be used in integer mode, which won't produce 5038 MHz.

    If the restrictions in Table 2 apply after IncludedDivide is adjusted for in SYNC mode, then it appears that there will be some frequencies which are simply not accessible in SYNC mode. Can anyone from TI confirm that this is the case?

  • 0 in reply to Tom Wallace
    TI__Guru** 108410 points

    Hi Tom,

    In this case, we have to lower the fpd or use a different MASH order to comply with the min. N restriction, otherwise, it won't lock. 

  • 0 in reply to Tom Wallace
    Prodigy 10 points

    For cases where the value of N is too low after adjustment for IncludedDivide, it appears to be necessary to set the input R-divider to a value greater than one so that N can be increased.

    The datasheet does not mention any restrictions on the pre-R or R-dividers for SYNC mode operation, only the input doubler and multiplier.