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LMK04821: LMK04821_SUPPORT

Lakshmi M B
Prodigy 40 points
Part Number: LMK04821

Hi,

We are using LMK04821 IC. With external clock frequency of 10Mhz, Is it possible  to generate SYSREF=120Mhz, DIVCLK= 122.88Mhz?. Kindly reply ASAP.

 

 

Thanks and regards,

Lakshmi M B

  • 0
    TI__Mastermind 29010 points

    Hello Lakshmi,

    LMK04821 is an integer PLL, so the least common multiple of the output frequencies must be the VCO frequency (or an integer divide of a VCO frequency). The least common multiple of 120 MHz and 122.88 MHz is 15,360 MHz. So simultaneous 120 MHz and 122.88 MHz output is not possible for any integer PLL with a VCO frequency less than 15.36 GHz. In other words, a single LMK04821 device cannot support this configuration.

    If it is possible to use two devices, one device could be used for each frequency. Then you could use nested 0-delay mode to establish a deterministic phase relationship between multiple devices. And if the 10 MHz reference is noisy, using two devices allows for two different VCXOs, which keeps PLL2 phase detector frequency high and minimizes jitter on clock outputs.

    Regards,

  • 0 in reply to Derek Payne
    Intellectual 855 points

    Hello Derek,

    Thanks for the reply.

    currently we are using 122.88MHz as reference input to LK04821 to generate JESD204B clock and Sysref.

    Dev_clk=122.88MHz

    Sysref= 120Khz.

    But if we need to apply an external clock for synchronization across several enodeB's we have to apply external clocks.

    two options, option one= 10MHz, option two = 30.72MHz.

    We have provided SMA connector to apply external clock to LMK04821.

    Kindly let us know whether we can generate 122.88Mhz on Dclk and 120KHz on SDclk. we need jesd clocks on 5 output pairs

    Regards,

    Sumathi

  • 0 in reply to Derek Payne
    Prodigy 40 points

    Hi Derek,

    PFA the file in which SYSREF and DIVCLK outputs are generated using external clock of 10Mhz and 30.72Mhz(CLKin1) .  Please verify the files and let us know if this is ok to use.

    Thanks and Regards,

    Lakshmi M BLMK_JUP_VALUES.rar

  • 0 in reply to Lakshmi M B
    TI__Mastermind 29010 points

    Hello Lakshmi,

    I cannot open that .rar file. Can you post the .tcs file? We recently enabled .tcs files on the whitelisted attachments.

    Regards,