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LMK04832: LMK04832 Pllatinum loop gain change principal

Zhizhao Niu
Intellectual 2315 points
Part Number: LMK04832

Hi team, 

I found when I simulation the difference VCXO gain value in LMK04832, there is an tip of "loop gain change is 51.9%" in Pllautinum and remind me to redesign the loop filter.  But I only change the Kv from 0.007MHz/V to 0.0081MHz/V, didn't change above 50%. So could you please help me understand why the loop gain change so much and need to redesign loop filter. 

Thanks. 

B.R.

Zhizhao

  • 0
    TI__Mastermind 29010 points

    Hi Zhizhao,

    Loop gain change is calculated as (Knew - Kold) / Kold.

    K1 = Kpd * Kvco / N = 0.45mA * 0.007MHz/V / 256 = 1.2304e-5

    K2 = Kpd * Kvco / N = 0.45mA * 0.0081MHz/V / 256 = 1.4238e-5

    (K2 - K1) / K1 = (1.4238 - 1.2304) / 1.2304 = 15.7%

    Since you're seeing a different loop gain change, there's two possible explanations:

    1. The coefficients looked very different the first time the "calculate loop filter" button was clicked. If you never clicked the "calculate loop filter" button, the loop gain value is initialized with the default parameters.
    2. There's a bug in the math. I checked but didn't see anything that would create a bug in the math.

    I went to the filter designer tab, and set the "forced component values" to the values you use in your loop filter, set the VCO gain to 0.007MHz/V, then clicked "calculate loop filter" button. LBW, PHI, and GAMMA are all restricted because the loop filter values are forced, so the loop characteristics do not change. However, clicking the button initializes the new baseline loop gain.

    Then I change the gain to 0.0081MHz/V:

    The correct change in loop gain is displayed.

    Regards,

    Derek Payne