
18 Filters and attenuators
18.1 Introduction to filters
A network which attenuates certain frequencies but passes others
without loss is called a filter. A filter therefore possesses at least one
'pass band', i.e. a band of frequencies where the attenuation is zero,
and at least one 'attenuation band', where the frequencies have
finite attenuation.
The frequencies that separate the various 'pass' and 'attenuation'
bands are called 'cut-off frequencies, usually denoted by
f\,fj,
etc.
or by /
c
if there is only one 'cut-off frequency.
All filters are constructed from reactive elements; otherwise the
attenuation would never become zero.
18.2 Th ...