Question about loop filter in PLL

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siboy

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The loop filter adds a unit gain buffer after the 2nd cap. What's the effect of the unit gain buffer?
 

To drive the second filter resistor with a low impedance. I guess the output to the filter is a charge pump or something that cannot drive a resistor directly.

But personally, I only use the first part of this compensation - pole zero pair with another high pole to kill off the gain at high frequency.
 

Thank you!

electronrancher said:
To drive the second filter resistor with a low impedance. I guess the output to the filter is a charge pump or something that cannot drive a resistor directly.
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Why a charge pump can't drive a resistor dircetly? The loop filter without the buffer is a 3rd order loop filter usuallly used in PLL.
 

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