A hybrid coil (or bridge transformer, or sometimes hybrid) is a

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

that has three windings, and which is designed to be configured as a circuit having four

ports A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...

that are conjugate in pairs. A signal arriving at one port is divided equally between the two adjacent ports but does not appear at the opposite port. In the schematic diagram, the signal into W splits between X and Z, and no signal passes to Y. Similarly, signals into X split to W and Y with none to Z, etc. Correct operation requires matched characteristic impedance at all four ports. Hybrids are a class of

directional coupler Power dividers (also power splitters and, when used in reverse, power combiners) and directional couplers are passive devices used mostly in the field of radio technology. They couple a defined amount of the electromagnetic power in a transmiss ...

in which the input port power is split equally between the two output ports. Forms of hybrid other than transformer coils are possible; any format of directional coupler can be designed to be a hybrid. These formats include

transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...

s and

waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...

Explanation

The primary use of a

voiceband A voice frequency (VF) or voice band is the range of audio frequencies used for the transmission of speech. Frequency band In telephony, the usable voice frequency band ranges from approximately 300 to 3400 Hz. It is for this reason th ...

hybrid coil is to convert between

2-wire In telecommunication, a two-wire circuit is characterized by supporting transmission in two directions simultaneously, as opposed to four-wire circuits, which have separate pairs for transmit and receive. The subscriber local loop from the telco tel ...

and 4-wire operation in sequential sections of a

communications Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...

circuit, for example in a four-wire terminating set. Such conversion was necessary when

repeater In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...

s were introduced in a 2-wire circuit, a frequent practice at early 20th century

telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

. Without hybrids, the output of one amplifier feeds directly into the input of the other, resulting in a howling situation (upper diagram). By using hybrids, the outputs and inputs are isolated, resulting in correct 2-wire repeater operation. Late in the century, this practice became rare but hybrids continued in use in

line card A line card or digital line card is a modular electronic circuit designed to fit on a separate printed circuit board (PCB) and interface with a telecommunications access network. A line card typically interfaces the twisted pair cable of a plain ...

Circuit diagrams

Hybrids are realized using

s. Two versions of transformer hybrids were used, the single transformer version providing unbalanced outputs with one end grounded, and the double transformer version providing balanced ports.

Single transformer

For use in

s, the single transformer version suffices, since amplifiers in the repeaters have grounded inputs and outputs. X, Y, and Z share a common ground. As shown at left, signal into W, the 2-wire port, will appear at X and Z. But since Y is bridged from center of coil to center of X and Z, no signal appears. Signal into X will appear at W and Y. But signal at Z is the difference of what appears at Y and, through the transformer coil, at W, which is zero. Similar reasoning proves both pairs, W & Y, X & Z, are conjugates.

Double transformer

When both the

and the 4-wire circuits must be

balanced In telecommunications and professional audio, a balanced line or balanced signal pair is a circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths and equal impedances to ground and to other ci ...

, double transformer hybrids are used, as shown at right. Signal into port W splits between X and Z, but due to reversed connection to the windings, cancel at port Y. Signal into port X goes to W and Y. But due to reversed connection to ports W and Y, Z gets no signal. Thus the pairs, W & Y, X & Z, are conjugates.

Applications

Telephone hybrid A telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network (PSTN) that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the a ...

s are used in

telephone exchange A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...

s to convert the 4-wire appearance to the 2-wire last mile connection to the subscriber's

telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...

. A different kind of hybrid is used in telephone

handset A handset is a component of a telephone that a user holds to the ear and mouth to receive audio through the receiver and speak to the remote party using the built-in transmitter. In earlier telephones, the transmitter was mounted directly on ...

s to convert the four wires of the transmitter (earpiece) and receiver (microphone) to the 2-wire line connection. This kind of hybrid is more commonly called an "induction coil" due to its derivation from high-voltage induction coils. It does not produce a high voltage, but like the high-voltage variety, it is a

step-up transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's c ...

in order to

impedance match In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize sign ...

the low-impedance carbon button transmitter to the higher impedance parts of the system. The simple induction coil later evolved into a form of hybrid as a

sidetone Sidetone is audible feedback to someone speaking or otherwise producing sound as an indication of active transmission. Sidetone is introduced by some communications circuits and anti-sidetone circuitry is used to control its level. Sidetone is exp ...

reduction measure, or volume of microphone output that was fed back to the earpiece. Without this, the phone user's own voice would be louder in the earpiece than the other party's.Lewis Coe, ''The Telephone and Its Several Inventors: A History'', pp. 124, 173, McFarland, 2006 . Today, the transformer version of the hybrid has been replaced by resistor networks and compact IC versions, which use

integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

electronics to do the job of the hybrid coil. Radio-frequency hybrids are used to split radio signals, including television. The splitter divides the

antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

signal to feed multiple receivers.

References

External links

* Douglas Rice (2008
Modelling Telephony Hybrids as 2 × 2 Matrices
{{Electric transformers Electric transformers Telecommunications equipment