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# LMK03328: How to design Loop filter

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

• Hi Mita,

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 ​​​​

• 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

Hi Team,

Can you give me nay feedback.

Mita

Clock and Timing Systems & Applications

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

• In reply to Hao Z:

Hao-san,

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

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