Radio frequency transmitters are used in combination with RF receivers. These devices are designed to compliment each other. One unit will transmit the radio frequency while the other unit will receive the signal and act accordingly. The application of a RF transmitter can be as simple as using it to control devices at home, for example, to lock the door using a remote device.
A basic RF transmitter for short-range transmissions can be easily devised using simple electronic components. Technological advances have made it possible for these transmitters to operate over large distances and across a broad frequency spectrum.
Technically, a transmitter is an electrical device, which can propagate electromagnetic signals like radio, television and other telecommunication devices, etc. A RF transmitter usually consists of major components like: an oscillator (which produces recurring electrical signals), a modulator (that enables the signal to be transmitted using a carrier signal), an amplifier (to improve the signal strength), and a power supply (which is essential for functioning of the components in a transmitter). RF transmitters are designed to produce continuously varying electric current, encode sine waves, and broadcast radio waves.
The modulator is the unit that modulates the signal information onto the carrier frequency, which is then broadcast through the airwaves by an antenna as radio signals. Some devices such as a cellular telephone often contain both a transmitter and a radio receiver. An electrical instrument that contains both units is referred to as a transceiver.
RF transmitters are used in many applications where broadcasting is done on a large scale. Even defense and military units use transmitters to exchange information. Modern day process control mechanisms, global positioning systems, surveillance systems, retail stores, inventory management systems, automatic vehicle locaters all use RF transmitters.
When you are selecting an RF transmitter it helps if you have an understanding of modulation methods such as AM and FM. In AM, the amplitude or the height of the signal is varied, while in FM, the instantaneous frequency of the wave is made to shift from the center frequency. On-off key (OOK) is the most basic type of modulation as it just consists of turning the signal on or off. An amplitude shift key (ASK) transmits the data by varying the amplitude of the transmitted signal. A frequency shift key (FSK) is a digital modulation scheme that uses two or more output frequencies. A phase shift key (PSK) is a digital modulation scheme where the phase of a transmitted signal is varied in accordance with the baseband data signal.
There are other factors you should consider when selecting RF transmitters including the supply voltage, the supply current, RF connectors, special features, and packaging. Some types of RF transmitters can come with visual or audible alarms or LED indicators which signal operating modes such as power on or reception. Other transmitters are made to attach to coaxial cables or include a connector or port that you can attach an antenna to. RF transmitters that are designed for outdoor use generally have a heavy-duty waterproof design. Transmitters that have internal calibration and a frequency range switch are also available.