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Phase continuity in LMX2531

Bob Haagensen
Intellectual 370 points
Other Parts Discussed in Thread: LMX2531, LMX2581

We're using an LMX2531 to transmit a 2.4 GHz carrier over a distance of 5 feet and measuring phase shift of the carrier as the LMX2531 is moved. The PLL is driven by an always-on 20 MHz TCXO. If the PLL is re-locked for any reason, we need the phase value to be the same when it re-locks. As long as the R divider is set to 1 (PD freq = 20 MHz) and the RF N divider is set to an integer value, this works as expected. However, if the R divider is set to anything other than 1 (and the N counter is re-adjusted appropriately) the phase value comes up with different, arbitrary values. Also, if the N counter is set to a non-integer value, the same problem occurs. Is this because the R frequency counters and fractional-N counters are not triggering from the same stage or some other phenomenon? Is there some way to get around it?

  • 0
    TI__Mastermind 28011 points

    Bob,

    If the R divider is not an integer, then there are multiple possible phase states.  For instance, when I divide 100 Mhz by 2, there are 2 possible phases I could get.  I think that this is the issue hat you are seeing with the R divider.

    With the fractional numerator,  think that the state of the accumulator might matter and this is adding some randomization to your phases. 

    I'm not sure there is a way around this one, but conside these programming bits that you can tinker with:

    PLL_N_RST (  R14[24] )  - Setting to 1 resets the N counter and holds it in reset

    PLL_R_RST (R14[25] ) - Setting to 1 resets th R counter and holds it in reset.

     

    So you could program register R14 and hold off on the LE signal and then use this as a synchronization pulse.  I'm not sure if it will wrk with a fraction or not.

     

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 120 points
    Hi Dean,

    I work with Bob.

    I don't see any reference to a register 14 in the data sheet. Is this undocumented?
    Also, the data sheet indicates that a register is only 20 bits long with 4 bits for the address. You suggested programming bits 24 and 25 of R14. Are the registers longer then 20 bits?

    Thanks,

    Matt
  • 0 in reply to Matt11
    TI__Mastermind 28011 points
    Thomas,
    My mistake, I was thinking about the LMX2581. For the LMX2531, its PLL_RST (R9[21])
    Dean