The capacitors C1 + C2 and C3 + C4 carry out the following functions:The R3 and R4 resistances lower the joint Q - factor. This is necessary because the high track input impedance in uncompensated track circuit's cause's excessive joint selectivity.The R1 (R2) resistance limits the low frequency current originating from the TE return. The current is closed using a transformer T1 even under equilibrium conditions between the rails. The transformer T1 (T2) could become saturated if this current is not low.The LP1 (LP2) is a cable pupin coil inductance. The tuning box is connected to the equipment (transmitter and receiver) using a quad cable.Cable capacity makes its characteristic impedance totally reactive.The cable has a pupin coil inductance to limit these problems.The characteristic impedance of the cable with the pupin coil is mainly resistive.For closer length cable the impedances can be well matched simply by using the transformer T1 (T2).
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