Newnes Audio and Hi-Fi Engineer's Pocket Book, 3rd Edition

298

Filters

Impedance The behaviour of a filter is determined by the

terminating impedance, so

it

is designed for a particular value

which is termed the design impedance. When so designed, the

impedance measured at the input terminals is the same as that

of the output and is called the iterative impedance. Filters may

then be cascaded providing the last one is terminated with the

correct impedance.

Constant-k low-pass filters These can be either T-type or

π-type having

a

nominal 18 dB/octave attenuation above the

cut-off frequency which is

-3

dB below the pass-band level.

Both types have the same properties, but as the π-type has only

one inducto ...