Editing Phase-locked loop (section)

===Filter===
The block commonly called the PLL loop filter (usually a low-pass filter) generally has two distinct functions.

The primary function is to determine loop dynamics, also called [[Nyquist stability criterion|stability]]. This is how the loop responds to disturbances, such as changes in the reference frequency, changes of the feedback divider, or at startup. Common considerations are the range over which the loop can achieve lock (pull-in range, lock range or capture range), how fast the loop achieves lock (lock time, lock-up time or [[settling time]]) and [[Damping factor|damping]] behavior. Depending on the application, this may require one or more of the following: a simple proportion (gain or attenuation), an [[integral]] (low-pass filter) and/or [[derivative]] ([[high-pass filter]]). Loop parameters commonly examined for this are the loop's [[gain margin]] and [[phase margin]]. Common concepts in [[control theory]] including the [[PID controller]] are used to design this function.

The second common consideration is limiting the amount of reference frequency energy (ripple) appearing at the phase detector output that is then applied to the VCO control input. This frequency modulates the VCO and produces FM sidebands commonly called "reference spurs".

The design of this block can be dominated by either of these considerations, or can be a complex process juggling the interactions of the two. The typical trade-off of increasing the bandwidth is degraded stability. Conversely, the tradeoff of extra damping for better stability is reduced speed and increased settling time. Often the phase-noise is also affected.<ref name=":0" />