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LMX2492: My question is about the LMX2492 Ramping Functions:

Part Number: LMX2492


Good Day. I have a customer who is working with LMX2492. Please see below his query for your reference. Thank you very much.

In the register R58[1] (RAMP_CLK) we can select an external clock (MOD bus) for the ramp increment. Is this same clock also used for the ramping state machine? In that case, will all ramp triggers be synchronized to this clock and therefore need to be longer than 1 or 2 periods of the ramp clock?

Best Regards,

Ray Vincent

  • Hi Ray,

    I cannot fully understand their question but in short, the ramp clock (internal or external) is used to increment the VCO frequency and therefore it determines the ramp time. All other logical execution, e.g. output a Flag signal, I believe is running on the internal state machine clock.

  • Hi Noel,

    Good Day. Please see below another query from our customer. Thank you very much.

    Maybe I can elaborate a little bit on my question. In our system we want to generate slow frequency ramps and thus need a slow external ramp clock. Also we want to trigger the ramp start with an external pulse. This works so far but we realized, that the trigger pulse must be longer than (at least) one ramp clock period. This is where the original question came from.
    I also have some follow-up question:
    Are there any specifications on the ramp clock, like a duty cycle requirement? Does it increment on rising or falling clock edge?
    Can the ramp clock be turned off in non-ramped operation? If so, is there some startup delay from ramp clock start to ramp start? In other words, can we also trigger the ramp clock start instead of the ramp start with the same result?

    Sorry for the wall of questions, but in the system some other functions must be synchronized to the ramp and thus exact timing specification is required. Thank you very much for taking the time!

    Best Regards,

    Ray Vincent

  • Hi Ray,

    The duty cycle of the external ramp clock should not be matter as the increment is happen with the rising edge. 

    Yes, I suggest turn off the ramp clock when not in ramp operation as this clock may add spurs to the PLL/VCO. We didn't measure, but there should be no (or very minimal) delay from ramp clock to ramp start as this is being processed in the logic. In fact, you can use the ramp clock to kick start the ramp.