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Why we still need to use a digital loop filter in a digital PLL?

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jihrenee

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For an analog PLL, the loop filter is to filter out high frequency jitter from the output of CP.
But for a digital PLL, why we still need to use digital loop filter?
I mean, for a typical digital PLL, its architecture is:
in_ref, in2--> TDC --> DLF --> DCO --> in2
The output of TDC is binary waveform (0101...)
why we still need a digital loop filter?
 

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