Transceivers and transponders are types of communication devices. They can operate with RF or optical technologies. They are used in various environments, from terrestrial to undersea and outer space. Basic transceivers and transponders provide two-way links in various types of networks. Specialized devices like muxponders increase available bandwidth, extend network reach and improve network efficiency.

This FAQ begins with a review of RF transceivers and transponders, looks at optical transceivers, transponders, and muxponders, reviews the uses of media converters, and closes with a look at how transponders are used for various identification applications, including how NASCAR uses them in motorsport.

RF transceivers
RF transceivers, often called radios, are a common and well-established technology area. They receive and demodulate RF signals and modulate and transmit signals. Some of the latest designs are software defined radios where components implemented in analog hardware in conventional radios, like mixers, filters, amplifiers, modulators, demodulators, and detectors, are implemented using software on a digital platform. Some key specifications for RF transceivers include data rate, sensitivity, output power, frequency, maximum transmission distance, and modulation technique. Common modulation techniques include:

• On-off key (OOK) is the simplest format and consists of simply turning the signal on and off to transmit information like Morse code.
• Amplitude modulation (AM) where the amplitude or height of the carrier wave varies to transmit the desired information, usually in an analog form such as audio signals.
• Amplitude shift key (ASK) varies the amplitude of the transmitted signal to send digital data. It’s used in various applications, from home automation to wireless base stations and tire pressure monitoring.
• Angle modulation, including frequency modulation (FM) and phase modulation (PM), where the frequency or phase, respectively, of the carrier wave are varied, often to transmit analog signals.
• Phase shift key (PSK) is a digital modulation technique that changes the phase of a constant frequency carrier wave to transmit digital information. Typical applications include wireless LANs, RFID, and Bluetooth.
• Frequency shift key (FSK) is a digital modulation technique that uses two or more frequencies to encode digital information. It’s used for low-frequency transmissions and low data rate applications like garage door openers.

Satellite transceivers vs. transponders
Transceivers and transponders are often found in satellite communications systems. According to the National Aeronautical and Space Administration (NASA), a transceiver consists of a co-packaged independent transmitter and receiver. While in a transponder, the received signal is used to derive the transmit carrier frequency. The frequency linkage in a transponder enables ground stations to recover Doppler shift information and determine the speed and range of the sending transponder without needing a separate (and power-consuming) ranging signal.

Communications satellites use multiplexed transponders to enable multiple sending sources to share a single channel. Communications satellites are essentially relay stations with dozens or hundreds of transponders, with a bandwidth of tens of MHz. Traditionally, satellite transponders operated on a ‘bent pipe’ principle and sent back directly what was received, only amplifying the signal and changing the frequency from uplink to downlink (Figure 1). Newer, ‘regenerative’ satellite transponders include some level of onboard processing. The signal is demodulated, decoded, and re-encoded in these designs before being sent back to Earth. While regenerative transponders are more complex and consume more energy, they can significantly improve the signal-to-noise ratio. They can implement selective processing, such as compression or multiplexing, of the data in the digital domain before retransmission.

Figure 1: Transponders in satellite communication systems can send back a copy or a modified version of the received signal. (Image: Intelsat)

Optical transceivers, transponders, and muxponders
While transceivers and transponders are commonly used in RF applications, muxponders are added to the mix in the optical communications domain. Optical transceivers are used in fiber optic networks to receive and transmit optical signals in a single module. Optical transceivers use a range of optical wavelengths, typically from 850 to 1,550 nanometers (nm), as well as different types of lasers, such as distributed feedback lasers (DFBs) and vertical-cavity side emitting lasers (VCSELs). There’s a wide array of standard optical transceiver formats. Some of the most common include gigabit interface converters (GBIC), small form factor pluggable (SFP), enhanced small form-factor pluggable (SFP+), 10-gigabit small form factor pluggable (XFP), and 100-gigabit small form factor module (CFP, C stands for 100 in roman numerals).

Optical transponders
Optical transponders are referred to as optical-to-electrical-to-optical (O-E-O) devices, wavelength converters, wavelength division multiplexers (WDMs), and fiber-to-fiber media converters. Whatever name they go by, these devices are generally used to extend network transmission distances or to match disparate optical networks. Depending on the application, optical transponders can retime signals, regenerate signals and reshape optical signals. They are defined according to their function, data rates, and transmission ranges. A typical optical transponder may receive signals, amplify them, and retransmit them on a different wavelength, at a different frequency, or both. For example, long-distance dense wavelength division multiplexing (WDM) networks use 10G/25G transponders for conversions like multimode to single mode fiber links, convert Ethernet to another protocol, or link dual fiber to single fiber connections. The general functions performed with optical transponders include one or more of the following (Figure 2):

• Multimode to single-mode conversion
• Creating redundant fiber paths
• Optical repeater
• Mode conversion
• Wavelength conversion

Optical muxponders
An optical muxponder is an advanced version of a transponder that can combine multiple wavelengths into a single stream using the optical transport network (OTN) protocol, as well as perform other functions. Its ability to multiplex multiple optical signals is like the transponders used in communication satellites. An optical muxponder can multiplex several low-capacity incoming signals into a single high-capacity outgoing fiber while maintaining the original signals’ data rate, wavelength, speed, and signal quality. They can enable longer-distance transmissions and increase network efficiency.

RF transponders for identification
In addition to the transponders used in communications satellites, another class of RF transponder is used in a variety of identification applications, including:

Aircraft – Identification of friend or foe (IFF) in military aircraft and secondary surveillance radar (beacon radar) used by air traffic control for general and commercial aviation. Secondary surveillance radar is used to supplement and overcome the limitations of primary radar systems. Primary systems don’t always work well for small aircraft; they can’t always overcome interference from hills, trees, weather, and other conditions and can have trouble estimating the aircraft’s altitude. Secondary radar systems interrogate the transponder on an aircraft providing more granular information about the plane and its flight path.

Marine – All ships with 300 or more gross tonnage and all passenger ships of any size on international voyages are required by the International Maritime Organization’s International Convention for the Safety of Life at Sea (SOLAS) to carry automatic identification system (AIS) transponders. AIS devices are transmitters and receivers but are called transponders since they transmit autonomously. The receiver sections of class B transponders on smaller vessels can be interrogated for information. A second type of marine transponder is radar beacons (RACONs) that make ships stand out more clearly on the radar screens of other ships. Sonar transponders are used for measuring distance underwater and supporting underwater position tracking, location marking, and navigation for various vessels, including submarines and autonomous underwater vehicles.

Automotive – The presence of a transponder in an ignition key is used as a form of security, preventing the operation of the car without the transponder signal. These transponders are a type of radio frequency identification (RFID) device. They are battery-less and are powered by the RF signal emitted from the automobile. The automobile interrogates the transponder and waits for a valid response before enabling the car to start. Transponders are widely used in toll road fee collection to speed up traffic.

Sport – Various active and passive transponders are used in track and off-road races to monitor the position and speed of automobiles, bicycles, and runners. Motorsport track systems often use cable loops buried in the track. For example, NASCAR uses a combination of RFID transponders and cable loops placed around the track to determine the positions of all cars during a caution period and ensure that relative positions are maintained until the race restarts. They are also used for lap timing and monitoring cars in and out of the pits (Figure 3).

Summary
Transceivers, transponders, muxponders, and media converters, including RF and optical technologies, are used in various applications. They are used for two-way and one-way communications, multiplexing signals, protocol conversions, providing redundant paths, and extending the reach of networks. The definition of a transceiver is standardized chiefly as a two-way communications device, while transceivers have a wider variety of definitions depending on their application.

References

Motor Sport Pro Timing Solutions, TAG Hauer
Optical Transceiver vs Transponder vs Muxponder: Which One is Right for Your Network?, Versitron
Optical Transponder, Fiber Optic Solutions
Satellite Basics, Intelsat
Transceiver vs Transponder: What Are the Differences?, FS