In applications where simultaneous full duplex communications is necessary, sometimes the transmission media is too expensive for separate transmit and receive lines or an antenna must be shared between transmit and receive signals. It is possible to use FDM, TDM, orthogonal coding or other methods to send information over the same link in two directions simultaneously.
However, there are also a few methods to send arbitrary analog and digital signals over the same link without special coding or modulation, and without the signals interfering significantly with each other. All these methods employ matched impedances and cancellation techniques to reject the transmit signal from being received locally. The principle of superposition is used to analyze these methods.
A circulator is a passive three or four port device, in which a radio or optical signal entering any port is transmitted only to the next port in rotation.
Figure 1 Circulator used to split transmit and receive from a bidirectional transmission line
A hybrid coil (or bridge transformer, or sometimes hybrid) is a transformer that is designed to have four ports in conjugate pairs.
Figure 2 Unbalanced to balanced hybrid
A signal arriving on one branch is divided between the two adjacent branches but does not appear at the opposite branch.
Figure 3 Fully balanced hybrid
An active hybrid employs a subtraction technique. The amount of transmit signal appearing at the receiver is predicted and subtracted. The subtracted portion can adjusted adaptively and incorporate delay as with an echo canceller.
Figure 4 Active 2 wire to 4 wire hybrid
This article presented a few of the methods which are employed to solve the bidirectional signal transmission and separation problem.