A transceiver circuit is electrical circuitry with the ability to both transmit and receive signals. Though transceiver use was originally confined to two-way radios in military and police applications, they now are incorporated into a wide range of consumer electronics, from lightweight consumer walkie-talkies and citizen's band (CB) radios, to cell phones, computer wireless networks, cordless telephones, high frequency (HF) and ultra-high frequency (UHF) radio transmitters, and more. Early forms of radio wave transceiver circuitry could only send signals or receive them, but not do both at the same time, known as simplex or half duplex circuits. Most modern transceiver circuits are duplex, however, allowing for simultaneous transmission of two signals over one channel, with reception of the signals at the same time.
Devices virtually identical to transceivers in function are transmitter-receivers, where separate circuitry exists inside a casing for each function of transmitting signals and receiving them. Transponders are another form of circuitry related to a transceiver circuit, where signals are transmitted and received simultaneously, but only in automated fashion, with one application being as a form of safety and identification beacon on aircraft. Transverters are another application of transceiver circuit technology. Amateur radio operators often use a transverter, which can convert HF or very high frequency (VHF) transceiver circuit signals to intermediate frequency (IF) ranges to amplify reception.
Using a transceiver circuit in the past meant carrying on audio conversations that required taking turns sending and receiving voice messages, and standard cell phones and radio frequency (RF) radios today allow constant back-and-forth voice transmission. As broadband transmission capability has advanced, fourth-generation (4G) smartphones and other devices now allow video transmission on transceiver circuits as well. If a transceiver circuit in a smartphone is in motion as someone rides on public transport or drives a car, maximum data transmission speed is 100 megabits per second (Mbit/s). A stationary user of a 4G transceiver-based device can send and receive signals at up to one gigabit per second however (Gbit/s), making video transmission practical on such devices for the first time in history. Such 4G transceiver circuit systems are also being built into laptop computers and other mobile devices as well.
Every time someone purchases a telecommunications device in one form or another that incorporates current technology, they are likely buying a transceiver circuit. Transceivers are at the core of most modern communications technology, and are built into everything from satellites to electronic keys for expensive automobiles that won't start unless the code transmission in the key housing is validated by the computer built into the car's ignition system. Even the radio frequency identification (RFID) chips built into some consumer product packaging to prevent theft, into the lining of US passports and certain credit cards, contain transceivers for decoding, along with transponders to continuously send out information on the card or passport for scanners to read.