506 BELL SYSTEM TECHNICAL JOURNAL 



relation of these impedances. This provides a method for permitting 

 low frequency signals to pass over the line without serious interference 

 from the rest of the network. 



The Twix 21 -Type Repeater Circuit 



A simplified diagram of a twin 21 -type circuit is shown in Fig. 15. 

 This consists of a line hybrid coil whose line windings are connected in 

 series with the line conductors and two 21 -type circuits. One pair of 

 terminals of one of the 21-type circuits is connected with the drop 

 winding of the hybrid coil which couples it effectively in series between 

 the two parts of the line. A network N s is connected to the remaining 

 terminals which balances the impedance of the two parts of the line 

 as seen from the 21-type circuit. One pair of terminals of the second 

 21-type circuit is connected to the bridge terminals of the hybrid coil 

 which bridges it across the line. A network A^^ is connected to the 

 remaining terminals which balances the impedance of the two parts of 

 the line as seen from the bridge. The internal connection of the series 

 21-type circuit is direct with respect to the line hybrid coil and the 

 bridged circuit is reversed. 



At first, assume that the potentiometers of the two 21-type circuits 

 are turned down and that a wave arrives at the W line terminals of 

 the twin 21-type circuit. At the peak of the positive half-cycle, cur- 

 rents will flow in the line hybrid coil in the directions indicated by the 

 arrows marked /. The passive impedances are chosen to fit the normal 

 impedances at the drop and bridge terminals of the line hybrid coil; 

 hence, none of this wave will reach the E end of the line. 



Next, turn the potentiometer of the series 21-type circuit up until 

 this circuit gives a gain. Due to the internal arrangement of this 

 circuit, an amplified current will flow in the line conductors in the 

 directions indicated by the large arrows marked Iqs. Little or none 

 of this current will reach the bridged circuit because of the balance 

 between the two parts of the line. 



Finally, turn the potentiometer of the bridged 21-type circuit up 

 until this circuit gives the same gain as the series circuit. Due to the 

 internal arrangement of the bridged circuit, amplified currents will 

 flow in the line conductors in the directions indicated by the arrows 

 marked Iqb- These currents are equal in magnitude to those caused 

 by the series 21-type circuit. In the line W the output currents annul 

 each other so that echoes returning toward the speaker are suppressed 

 while the currents in the line E co-operate and an amplified wave travels 

 over the line E to the listener. 



