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SONET and Ethernet clock synthesizer with frequency hold-over

Mahesh KC03
Intellectual 1360 points
Other Parts Discussed in Thread: LMK04808, LMK04906, LMK03328

Hi,

I am trying to promote our clocking solution at one of my customer who does optical back hauls based on SONET standard.

The situation is that the reference clock comes from the port and we need to clean it and generate 2 frequencies mainly: 156.25 MHz (Ethernet) and 155.52 MHz (SONET) with less than 1ps jitter. There should be a flexibility to generate both the values on one output pair. That is to say, the user must be able to choose between  Ethernet and SONET clock on the same line by changing the output divider setting.The most important thing is the hold-over option. The output frequency must be held in spite of the absence of the  reference input .

I checked LMK04808, LMK04906 as they has hold-over but it lacks fractional divider required to generate both the frequencies on the same line.

Do we have any solution for this? 

  • 0
    TI__Mastermind 32575 points
    The LMK03328 clock gen can generate the 2 clock domains with output mux to switch between the two frequencies, but you'd need to use a separate jitter attenuator chip that supports the holdover and reference switchover requirements before the clock gen.

    Alan
  • 0
    TI__Mastermind 24205 points

    Hi Mahesh,

    You can use LMK04906 with a 25 MHz VCXO reference. In order to generate 156.25 MHz, you will need to run the VCO at 2500 MHz and for achieving 155.52 MHz, you can run the VCO at 2488.32 MHz (PDF for PLL2=40 kHz). The performance of the output clock in both cases should meet 1ps (Integrated RMS Jitter 12kHz - 20 MHz). However, you will need to re-program the PLL2_N and PLL2_R divider which will trigger a VCO recalibration while switching from Ethernet domain to SONET domain. Can the system handle a temporary glitch when PLL2 recalibrates to switch between these output frequencies? Holdover is a function of PLL1 which will not change between the two scenarios.

    Regards

    Arvind Sridhar

  • 0 in reply to Arvind Sridhar
    TI__Intellectual 1360 points
    Thank you team.
    I am doubtful about the customer acceptance of our 2 chip solution. However, I will get a feedback from him.