With ideal components there is no need to use resistors in the design of lattice filters. However, practical considerations of properties of real components leads to resistors being incorporated. 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. This ensures that the attenuation at high frequency is the same as the attenuation at low frequency and brings the filter back to a flat response. The purpose of the shunt resistors, R2, is to bring the image impedance of the filter back to the original design R0. The resulting filter is the equivalent of a box attenuator formed from the R1's and R2's connected in cascade with an ideal lattice filter as shown in the diagram.
Read more about this topic: Lattice Phase Equaliser
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