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LMK04828: Clock Output/Configuration Issues

Michael Brewer
Prodigy 20 points
Part Number: LMK04828
Other Parts Discussed in Thread: ADC12DJ5200RF, , LMK04832

Hi! 

I am working on configuring the ADC12DJ5200RF dev board and having some difficulties programming the LMK04828B. I am using TICS Pro to configure each register with the appropriate values, then export that hex register data into our script that programs the board. 

My issue is this:

The 'VCO/Clock Dist. Frequency' using VCO1 is limited between 2920 and 3080 MHz. I am constrained by what I need the clock outputs to be in order to drive and interact with peripherals, most importantly 320 MHz off one of the clocks. I cannot seem to find a configuration that maps VCO1 in the desired range and also gives me a clock output that lands clean on 320 MHz.

My question:

Is this VCO1 range absolutely firm? What exactly happens when you program the VCO1 clock outside of this range? 

I tested a bunch and saw that when I configured VCO1 to 3000 MHz, all clock outputs probed cleanly and at the expected frequencies.

When I used the same configuration and just changed the input clock to 320, the VCO1 out follows to 3200. When implementing this configuration and probing, I found the clock output frequencies were all out of whack.


Looking for some insight and assistance handling this.

Thanks in advance.

  • 0
    TI__Mastermind 34270 points
    Michael Brewer said:
    Is this VCO1 range absolutely firm? What exactly happens when you program the VCO1 clock outside of this range?

    The VCO range in the datasheet (and provided by the software) is the range across which VCO operation is guaranteed at all operating temperatures within recommended operating conditions.

    In practice, the VCO is an L-C tank filter, with internally adjustable C component. The exact values of L and C can vary by some amount due to process and temperature variation, so not every unit will share the same characteristics. But in general, there's some "padding" on the total VCO range - maybe a few tens of MHz - which allows for process variation without impacting guaranteed range. For limited production applications where every device can be screened across temperature, it is plausible to operate with a minimal excursion outside of the VCO range, but the probability of encountering a unit with a coverage gap (particularly across temperature) goes up as the frequency excursion increases, until at some point no unit can cover the desired frequency.

    I suspect you will not find many LMK04828 units that can cover 3200MHz on VCO1. This is over 100MHz outside of the nominal upper frequency limit, and probably introduces some weird effects to the LMK04828.

    I note that VCO0 includes 2560MHz, which could yield 320MHz with divide-by-8.

    If this isn't compatible with the rest of your frequency plan, and you really need 3200MHz, I recommend swapping your LMK04828 with a p2p-compatible LMK04832, which includes 3200MHz in the coverage range. Aside from the extended frequency, there are a number of other advantages offered by LMK04832, including more output routing flexibility, higher PLL2 phase detector performance, and larger output divider range.

  • 0 in reply to Derek Payne
    Prodigy 20 points

    Understood, thank you so much for clearing that up! 
    Also, good idea using VCO0 to find a multiple that gives 320.