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LMK04832-SP: Unreachable Frequencies?

Daniel Tovstyka
Prodigy 20 points
Part Number: LMK04832-SP
Other Parts Discussed in Thread: LMK04832, LMK04821, LMK04133, LMK04033, , LMX2615-SP

For my design, I am trying to have the LMK04832 produce clock signals of speeds 1 GHz and 2 GHz. Now, the internal VCO0 produces frequencies in the range between 2440 and 2600 MHz, and VCO1 produces frequencies in the range between 2945 and 3225 MHz. These then are divided by whole integers into the 14 clock outputs. My question is because the clock dividers only divide by whole numbers, can I reach a frequency of 2 GHz? By my math, frequencies between 2.44 GHz and 1.6275 GHz (= 3.225 / 2) are unreachable. Or am I missing something in the datasheet.

  • 0
    TI__Guru* 97180 points

    Hi Daniel,

    to get both of these frequencies out at the same time, the VCO frequency must be an integer multiple of 2000MHz (for integer-N PLL). As such, LMK04832 cannot support.

    Do you need jitter cleaning function? LMK04821, LMK04033 and LMK04133 can support these frequencies.

    If not, will you consider to use synthesizer to generate these frequencies? 

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Hey Noel,  

    Thank you for your response! For our design, majority of our parts have to be radiation-tolerant. Are there any radiation-tolerant synthesizers you would recommend?

    I am looking at the LMK04821, and it looks almost identical to the LMK04832-SP. Would you happen to know if they use the same die? 

  • 0 in reply to Daniel Tovstyka
    TI__Mastermind 31710 points

    Daniel,

    LMK04821 is not radiation-tolerant, it's a different design.

    LMK04832-SP does support an external VCO up to 6.4GHz on Fin0 port, or up to 3.25GHz on Fin1 port. If you have your own VCO at 2GHz or 4GHz (there is a mandatory divide-by-2 block in Fin0 for inputs above 3.25GHz, so you could not use a 6GHz VCO), you could still use LMK04832-SP with external VCO.

    As an alternative, LMX2615-SP could produce a 2GHz clock output. But the channel divider is shared between its two outputs, so you would be unable to produce the 2GHz and 1GHz clocks simultaneously without using some external divider. TI does not (yet) have a radiation-tolerant external divider which could simplify this use case at 2GHz, so you would need to look elsewhere for such a component; that said, radiation-tolerant divide-by-2 ICs certainly exist (e.g. ADH361S).

    If you need better performance than an external VCO can provide, you could use LMX2615-SP to synthesize a 2GHz signal, and drive the LMK04832-SP in distribution mode as a buffer/divider for your simultaneous 2GHz and 1GHz clocks. This is more complex than just using an external VCO and would cost considerably more, but the performance of LMX2615-SP VCO is very good and may be better than available options at 2GHz or 4GHz.

    Regards,

    Derek Payne