Lattice phase equaliser
The lattice phase equaliser was invented by Otto Zobel[1][2] using a filter topology proposed by George Campbell.
[3] The characteristic impedance of this structure is given by and the transfer function is given by The lattice filter has an important application on lines used by broadcasters for stereo audio feeds.
The same is true of the absolute phase distortion on each leg (left and right channels) of a stereo pair of lines.
Many other types of filter section are intrinsically unbalanced and have to be transformed into a balanced implementation in these applications, which increases the component count.
This second method may not only make calculation easier but it is also a useful property where lines are being equalised on a temporary basis, for instance for outside broadcasts.
An alternative, and possibly more accurate, view of this filter's response is to describe it as a phase change that varies from 0° to 360° with increasing frequency.
Sections designed to equalise low audio frequencies will have larger inductors with a high number of turns.
This results in significant resistance being in the inductive branches of the filter, which in turn causes attenuation at low frequencies.
In the example diagram, the resistors placed in series with the capacitors, R1, are made equal to the unwanted stray resistance present in the inductors.
The purpose of the shunt resistors, R2, is to bring the image impedance of the filter back to the original design R0.
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.
This circuit was first used by George Washington Pierce, who needed a delay line as part of the improved sonar he developed between the world wars.
The circuit can be considered a low-pass m-derived filter with m > 1, which puts the transmission zero on the jω axis of the complex frequency plane.
