1. The ADPLL filter is needed to remove any unwanted high frequency components which might pass out of the phase detector and appear in the DCO tune line.
2. If the filter is not designed correctly then oscillations can build up around the loop, and large signals will appear on the tune line.
3. Without a loop filter you would almost certainly be feeding a waveform rather than a steady voltage back to the DCO and the result would be unpredictable and unstable at the very least. If the loop time constant is too short you will get residual signal on the control voltage causing FM on the DCO but the reaction (lock) time would fast.
2. If the filter is not designed correctly then oscillations can build up around the loop, and large signals will appear on the tune line.
3. Without a loop filter you would almost certainly be feeding a waveform rather than a steady voltage back to the DCO and the result would be unpredictable and unstable at the very least. If the loop time constant is too short you will get residual signal on the control voltage causing FM on the DCO but the reaction (lock) time would fast.
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