Phased Locked-Loop is an important building block of linear systems. Electronic
phased locked-loop came into in 1930s when it was used for radar communications and
communication applications. The high cost of realizing PLL in discrete form limited its use
earlier. Now with advanced IC technology PLLs available as inexpensive monolithic ICs, this
technique for electronic frequency control is used today in satellite communication systems, air
borne navigation systems, Computers etc