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Original question:

LMK04826: Questions about PLL2

LMK04821: VCO calibration in Zero-delay mode

ken mason
Prodigy 205 points

Part Number: LMK04821

Hi

I'm using  PLL2 (single loop)  in zero delay mode running at 2GHz, 40MHz TCXO input but want to use 80MHz PD frequency for best phase noise. This means I can't use the main FB path as it has a fixed divide by 2 in the FB path, thus the resolution is 160MHz and this does not work with 2GHz. So I decided to use zero delay mode and use the DCLK6 divide as this has no prescaler and supporst a division of 25 (gives 80MHz).

I'm using the TICS Pro software and all seems Ok apart from an error related to calibration, clearly the calibration cannot use the N-divider values because this would result in the wrong VCO frequency (due to the presence of the prescaler), the only way I see it can work is if the calibration is first done without zero delay (i.e. PD frequency =40MHz) and then switched to zero delay.

some advice would be appriciated

regards

Ken

I've attached a screen shot 

  • 0
    TI__Mastermind 35330 points

    Ken,

    The N Prescaler value can be modified to any value in the range [2,8]. 2000/80 = 25, so you could configure the N prescaler for 5 and the N Cal divider for 5.

    As a side note, there are sometimes cases where you really can't achieve the divide required. Supposing the total N-divide was to be 23, it isn't possible to achieve with the allowed prescaler values. In these cases, you do exactly as you said: set the phase detector a factor of 2 lower to ensure a valid prescaler can be achieved (i.e. double the R-divide and the N Cal-divide), write the N-divider LSBs with the value used in zero-delay mode to calibrate the VCO, then restore the R-divider to the doubled phase detector value. In your case, you could just disable/enable the doubler.

    Regards,