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LMK05318B: Proposed architecture: Drive XO input from the uBlox LEA-M8F 30.72MHz frequency reference signal

Quint Underwood
Intellectual 380 points
Part Number: LMK05318B

Tool/software:

Hello,

I would like to verify a high-level proposed architecture before producing PCBs.

Background:

Our embedded system contains a uBlox LEA-M8F GPS module, which has both a standard 1PPS timing reference signal (derived from GPS), as well as a 30.72MHz disciplined frequency reference derived from a VCTCXO & PLL internal to the GPS module (clock specs here).

Proposal: Feed the uBlox 1PPS signal into the PRIREF input of LMK05318B, and feed the uBlox 30.72MHz signal into the XO input of the LMK05318B. Upon loss of GPS lock, the 30.72MHz clock enters holdover mode until GPS lock is reacquired. 30.72MHz is one of the recommended XO frequencies, we will verify levels match, ensure correct transmission line termination, etc. I've already created the configuration in TICS PRO that meets all of our input/output frequency requirements, so I'm mostly double checking there's not some obscure requirement I'm missing (e.g. that the XO and PRIREF be derived from separate sources, etc.).

Any feedback is appreciated, thank you.

  • +1
    TI__Genius 16585 points

    Hi Quint,

    To get 1PPS to properly lock, we recommend using a low frequency and high stability XO input. For example, a 12.8 MHz TCXO or OCXO.This is because the LMK05318B uses a PPS detector that is dependent on the XO frequency accuracy. To determine the max XO ppm allowed by a given XO input frequency, use the following formula:

    • max XO ppm = 1PPS cntr value / XO frequency (in MHz)
    • The max PPS threshold counter value (R243) is 63 as shown in the TICSPRO GUI screenshot.
    • This means that with an XO input frequency of 30.72 MHz, the max XO input ppm allowed is...
      • 63 / 30.72 = 2.05 ppm
    • If I understood the datasheet correctly, your XO input of 30.72 MHz has an accuracy of 100 ppb which is much lower than 2.05 ppm. So I see no issues with using the 30.72 MHz signal as the XO input.

    A questions for you,

    1. Does your design require only one DPLL reference (PRIREF) or is another DPLL reference required as well? In other words, will both PRIREF and SECREF be used?
    2. Will the 30.72 Mhz clock always be present? The XO input to the LMK05318B must always be present to avoid a glitch or loss of clock output. The PRIREF input can come and go but the XO input must always be there for undisturbed output clocks.

    Regards,

    Jennifer

  • 0 in reply to Jennifer Bernal
    Intellectual 380 points

    Hi Jennifer, thank you for your detailed response. To answer your questions:

    1. We only expect to use PRIREF, at least for now. Perhaps it evolves in the future.

    2. The 30.72MHz clock is always present (assuming GPS unit is powered on), regardless of GPS lock. That OSC enters holdover mode when there is no GPS lock.