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LMK03328: How to design Loop filter

Expert 1755 points

Replies: 6

Views: 104

Part Number: LMK03328

Hi Team,

 

Can you advise me my question bellow.

 

I tried to tune loop filter of LMK03328 referring the document

titled “8508.PLL Performance, Simulation, and Design (2).PDF”.

The equation 9.1 says Fc is proportional to the square root K.

Can you let me know how this relationship is concluded?

 

Please let me know if my thought bellows are correct or not

   G(s) = ( K * Z(s) )/ s

        K : products of the phase detector gain and VCO gain

         Z(s) : Loop filter impedance

To get the frequency Fc on which the G(s) = 1

   1 = ( K * Z(s) )/ s

It can be reformed as bellow

2Pi*Fc = K * Z(Fc)

The second order filter consist of Cp and Rs+Cs.

If Cs >> Cp, it may be almost equal to Rs +Cs

       2Pi*Fc = K *(Rs + 1/2PI*Fc*Cp)

     (2Pi*Fc) **2 = K *(2PI*Fc*Rs + 1/Cs)

From this equation, I understand that Fc is proportional to Rs and

invers-proportional to the square root of Cs.

However, I cannot say that Fc is proportional to the square root of Cs,

because K has two terms, 2PI*Fc*R and 1/Cs.

How you can conclude that Fc is proportional to the square root K.

Mita

  • Genius 12550 points

    Hi Mita,

    In general the loop filter can be designed through PLLatinum sim, which can be downloaded for free: https://www.ti.com/tool/PLLATINUMSIM-SW

    You can then select the device and start the design. The tips can be found after clicking the little question marks in the interface.

    For PLL theory, I recommend studying Razavi's RF micorelectronics. 

    Regards,
    Hao

    Clock and Timing Systems & Applications

    To view training videos on Clock and Timing Solutions please visit  TI Precision Labs ​​​​

    More information Clock and Timing System products: http://www.ti.com/clock-and-timing/overview.html

  • In reply to Hao Z:

    Hao-san,

     

    Thank you for our response.

    I know that PLLatimum can be used to design the loop filter.

     

    However, I had chance to take look at the training video.

    In this video, Simon-san stated that Fc is proportional to square root of K.

    I cannot fully understand it and ask this question.

    Can you explain how you can get this relationship.

    https://training.ti.com/jp/engineer-it-how-design-excellent-pll-vco-noise-performance

     

    Mita

  • In reply to user4779049:

    Hi Team,

    Can you give me nay feedback.

    Mita

  • Genius 12550 points

    In reply to user4779049:

    Hi Mita-san,

    Please see page 92 of this document: https://www.ti.com/lit/ml/snaa106c/snaa106c.pdf

    Regards,
    Hao

    Clock and Timing Systems & Applications

    To view training videos on Clock and Timing Solutions please visit  TI Precision Labs ​​​​

    More information Clock and Timing System products: http://www.ti.com/clock-and-timing/overview.html

  • In reply to Hao Z:

    Hao-san,

    Thank you for your feedback.

    At last, please let me confirm my understanding.

    Does " if one neglects all poles and zeros" means the equation 12.15 can be

    transformed to the equation bellw?

    (G(S) := K / A0 * s**2 and   assuming that G(s) = 1,

    S**2 = K/A0

    Is my though correct?

    Mita 

  • In reply to user4779049:

    Mita-san,

    Yes, this is the idea.  The approximations are:

    1+s*T2=1

    1+s*T1=1+s*T3=1+s*T4 = 1

    Regards,

    Dean

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