Radio Frequency by Steve Winder and Joe Carr - HTML preview

3.4 Balanced line hybrids

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.