A lattice phase equaliser or lattice filter is an example of an all-pass filter. That is, the attenuation of the filter is constant at all frequencies but the relative phase between input and output varies with frequency. The lattice filter topology has the particular property of being a constant-resistance network and for this reason is often used in combination with other constant resistance filters such as bridge-T equalisers. The topology of a lattice filter, also called an X-section is identical to bridge topology. The lattice phase equaliser was invented by Otto Zobel. using a filter topology proposed by George Campbell.
The characteristic impedance of this structure is given by;
and the transfer function is given by;
... The lattice phase equaliser cannot be directly transformed into T-section topology without introducing active components ... Transformer action can be conveniently achieved in the low-in-phase T-section by winding both inductors on a common core ... response of this section is identical to the original lattice, however, the input is no longer constant resistance ...
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