Antiside Tone Circuit

Antiside Tone Circuit Points : Antiside Tone Circuit telephone To control the level of the side tone to the desirable amount anti-side tone induction coil (A.S.T.I.C) is used in the subscriber’s telephone set. The anti-side tone induction coil serves the following;
a. Control the level of the side tone to the desirable amount.
b. Ensures that no dc current flows through the receiver.
c. Matching between the impedances of the transmitter and receiver with that of the line.

A simple circuit arrangement for the reduction of side tone is shown.
The principle of the arrangement can be explained as follow:
Consider the figure (A) in this case the transmitter is transmitting. If Z_B = Z_L and L_I = L₂, then the transmitter current divides equally in L₁ and L₂. The magnetic field produced by these two windings will be equal and opposite and hence cancel each other’s effect, thus no emf is induced in L₃ and the side tone is completely eliminated. Practically L₁ and L₂ are not exactly equal and hence some emf is induced in L₃, thus the receiver receives a portion of transmitter current. In figure. (B) the receiving current passes through L₁ and L₂, the magnetic field produced are in the same direction and reinforced each other’s effect. An emf is induced in the receiver circuit and current flows through the receiver.

The anti side tone circuit is modified, to make it more practical and this modified circuit is shown.
The three windings L₁, L₂ and L₃ are magnetically coupled. L₁ and L₂ are not equal and hence an emf is induced in L₃ from the transmitter. The value of R, across which the receiver is connected, is such that the voltage drop across it is equal and opposite to the emf induced in L₃. This results in a much reduced side tone in the receiver.