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Part Number: LMK03328
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.
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.
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
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In reply to Hao Z:
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.
In reply to user4779049:
Can you give me nay feedback.
Please see page 92 of this document: https://www.ti.com/lit/ml/snaa106c/snaa106c.pdf
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?
Yes, this is the idea. The approximations are:
1+s*T1=1+s*T3=1+s*T4 = 1
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