RF Transmitters Information
An RF transmitter is a device that creates a radio signal containing information that with the use of an antenna propagates some distance. The earliest practical RF transmitter was known as a spark gap transmitter which simply generated a spark between two electrodes to create the RF signal. Much like lightning creates static on a radio, the spark gap created a brief moment of static on a receiver. This method was used in conjunction with Morse code to send information via pulses.
Modern RF transmitters, while quite different, are based on the same principles as the spark gap transmitter. Instead of a spark gap, modern transmitters use transistors or tubes. This circuitry creates an oscillating RF signal at a specific frequency or range that is carried to an antenna which is tuned to match the frequency.
Selecting RF Transmitters
RF transmitters carry information in a number of different ways. Where spark gap transmitters carried information with a method of simply turning the signal on and off, called on off key (OOK,) modern transmitters do so in more complex ways.
- Amplitude modulation (AM) causes the baseband signal to vary the amplitude or height of the carrier wave to create the desired information content.
- Frequency modulation (FM) causes the instantaneous frequency of a sine wave carrier to depart from the center frequency by an amount proportional to the instantaneous value of the modulating signal.
- Amplitude shift key (ASK) transmits data by varying the amplitude of the transmitted signal.
- Frequency shift key (FSK) is a digital modulation scheme using two or more output frequencies.
- Phase shift key (PSK) is a digital modulation scheme in which the phase of the transmitted signal is varied in accordance with the baseband data signal.
Many transmitters today are focused on data transmission and are being increasingly pushed to get more and more information throughput. Since a single radio frequency can only carry a finite amount of data at a time, often a system of utilizing a range of frequencies called spread spectrum is used. Also being utilized are higher frequencies, since the higher the frequency the more bandwidth is possible. However, this method comes with other drawbacks relating to range.
Applications and Industries
RF transmitters are used in a variety of applications and industries. Often, devices that are used with integrated circuits (ICs) incorporate surface mount technology (SMT), through hole technology (THT), and flat pack. In the telecommunications industry, RF transmitters are designed to fit in a metal rack that can be installed in a cabinet. RF transmitters are also used in radios and in electronic article surveillance systems (EAS) found in retail stores. Inventory management systems use RF transmitters as an alternative to barcodes.
Additional considerations when selecting RF transmitters include supply voltage, supply current, RF connectors, special features, and packaging. Some RF transmitters include visual or audible alarms or LED indicators that signal operating modes such as power on or reception. Other devices attach to coaxial cables or include a connector or port to which an antenna can be attached. Typically, RF transmitters that are rated for outdoor use feature a heavy-duty waterproof design. Devices with internal calibration and a frequency range switch are also available.