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CDCM6208 GUI gives negative PPM

Hello,

I setup the CDCM GUI with my clock sources, it seemed to work well but one of my clocks had a negative PPM reading......is this ok?

Primary input 25MHz, sec input disabled, 1.8V all channels

Y4 LVCMOS 25MHz

Y5 LVCMOS 133.3MHz

Y0 - Y3 LVDS 125MHz

Y6 LVCMOS 100 MHz

Y7 LVCMOS 100MHz

  • Hi Kevin,

    This negative ppm reported by the tool is correct. Is your frequency 133.3 MHz or 133 + (1/3) MHz? If your target frequency is 133+(1/3) MHz, a 0 ppm solution can be achieved

    Regards

    Arvind Sridhar

     

  • What does a negative PPM mean? How is that correct?

    I put the same frequency outputs in the tool but i started with 133.333 and then changed to 133.33 and then 133.3 and now i have 0.496419 PPM.......not sure why i'm getting different numbers with the same frequencies now.

    Also how come i can't look at the phase noise plot for Y5?

  • Hi Kevin,

    Negative ppm on the tool implies that the computed (actual) frequency is less than the desired (target) frequency. For instance if the desired frequency is 100 MHz and the tool achieves 99.999 MHz, the ppm error reported is

    (Target - Actual) / Target * 1000000

    = (99.999-100) / 100* 1000000 = -10ppm

    If the Actual frequency is computed as 100.001 by the tool the ppm error is reported positive

    (Target - Actual) / Target * 1000000

    = (100.001-100) / 100* 1000000 = 10ppm

    Every time you change the desired (Target) output frequency, the tool is trying to get as close as possible to the Target frequency while maintaining the ppm limits to within the tolerance specified in the "Frequency Planner" (+/-50ppm nominal) assuming "Best Jitter" setting is selected. If the "Best PPM" option is selected, the smallest ppm error (positive or negative) is reported.

    Also, phase noise is only available at this time on the outputs that are generated using the Integer Divider. Fractional divider outputs are not modeled. For Fractional Divider performance, please refer to Page 19 (Typical Characteristics) in the datasheet.

    Regards

    Arvind Sridhar