A system and methods for generating a variable rate filtered output using synchronous
filters having the same filter sampling time, that avoids complexities of asynchronous
filters. A system employs multiple filters that are staggered in time and set so
that the output of one of the multiple filters is available whenever a secondary
process requires state information. In another embodiment, the synchronous filters
are programmable so as to change the filter sampling time. This configuration is
possible when it is known that the prescribed time interval of the secondary process
is longer than the filter sampling time and the prescribed time interval is an
integer multiple of the filter sampling time. By using programmable filters, the
number of filters required to accommodate a certain prescribed time interval can
be minimized. Whether a system employs multiple non-programmable filters staggered
in time or programmable filters, the invention achieves producing a variable rate
filtered output while using synchronous filters. Thus, computationally intensive
and complicated variable rate filters can be avoided while minimizing processor
requirements. In another aspect of the invention, the inventive method of generating
a variable rate filtered output is employed in a lithography system to control
the movements of various components therein. In one embodiment, the filtered outputs
are used to estimate the relative positions of the reticle and the wafer. With
the variable rate feature, the lithography system can dynamically accommodate for
the noise arising from component movements and other sources in the wafer exposure process.
