Radio transmitters and receivers are often controlled over a two-wire line. To facilitate this a balanced line hybrid circuit, consisting of two transformers connected back to back as in Figure 3.2, is inserted between the transmitter and receiver, and the line.
A signal from the receiver audio output is fed to winding L1 of transformer T1 which induces voltages across L2 and L3.The resultant line current also flows through L4 and produces a voltage across L6 which would appear as modulation on the transmitter but for the anti-phase voltage appearing across L5. To ensure that the voltages cancel exactly a variable resistor, and often a capacitor to equalize the frequency response, is connected between L2 and L5.
A signal arriving via the line is applied to the transmitter as modulation; that it is also applied to the receiver poses no problem.Balance control