In a feedback system such as a PLL, the integrating function associated with a loop filter capacitor is instead implemented digitally and is easily implemented on the same integrated circuit die as the PLL. There is no need for either an external loop filter capacitor nor for a large loop filter capacitor...http://www.google.com/patents/US20040051590?utm_source=gb-gplus-sharePatent US20040051590 - Digitally-synthesized loop filter circuit particularly useful for a phase locked loop
