- Tuned radio frequency receiver
A tuned radio frequency receiver (TRF receiver) is a
radio receiverthat is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. A 3 stage TRF receiver includes a RF stage, a detector stage and an audio stage. Generally, 2 or 3 RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage. The detector converts RF signals directly to information, and the audio stage amplifies the information signal to a usable level. Prevalent in the early 20-th century, it can be difficult to operate because each stage must be individually tuned to the station's frequency. It was replaced by the Superheterodyne receiverinvented by Edwin Armstrong.
The TRF receiver was patented in 1916 by
Ernst Alexanderson. His concept was that each stage would amplify the desired signal while reducing the interfering ones. The final stage was often simply a grid-leak detector.
The significance of the term "tuned radio frequency" is best understood when compared to the
Superheterodyne receiver. A tuned radio frequency receiver actually tunes the receiver on the true radio frequency whereas the Superheterodyne receiver, tunes the desired signal after conversion to an intermediate frequency. Many homemade radios constructed by enthusiasts today, are tuned radio receivers, and these can range from single stage to multi-stage receivers.
A problem with the TRF receiver is that interelectrode capacitance causes
oscillations and other modes in the tuned circuits. In 1922, Louis Alan Hazeltineinvented the neutrodynecircuit, which - as its name implies - neutralizes these capacitances.
Antique TRF receivers can often be identified by their cabinets. They typically have a long, low appearance, with a flip-up lid for access to the
vacuum tubes and tuned circuits. On their front panels there are typically two or three large dials, each controlling the tuning for one stage. Inside, along with several vacuum tubes, there will be a series of large coils. These will sometimes be tilted slightly to reduce interaction between their magnetic fields.
Disadvantages of TRF receiver
They have 3 distinct disadvantages that limit their usefulness to single-channel, low-frequency applications. The primary disadvantage is their bandwidth is inconsistent and varies with
center frequencywhen tuned over a wide range of input frequencies. This is caused by a phenomenon called the "skin effect". At radio frequencies, current flow is limited to the outermost area of the conductor; thus, the higher the frequency, the smaller the effective area and the greater the resistance. Consequently, the "quality factor" (Q=XL/R) of the tank circuits remains relatively constant over a wide range of frequencies, causing the bandwidth (f/Q) to increase with frequency. As a result, the selectivityof the input filter changes over any appreciable range of input frequencies. If the bandwidth is set to the desired value for low-frequency RF signals, it will be excessive for high-frequency signals. The second disadvantage is its instability due to the large number of RF amplifiers all tuned to the same center frequency. High-frequency, multistage amplifiers are susceptible to breaking into oscillations. This problem can be reduced somewhat by tuning each amplifier to a slightly different frequency, slightly above or below the desired center frequency. This technique is called "stagger tuning". The third disadvantage is their gainsare not uniform over a very wide frequency range because of the non-uniform L/C ratios of the transformer-coupled tank circuits in the RF amplifiers.
1. Wayne Tomasi, “Electronic Communications Systems: Fundamentals Through Advanced”, 5th edition, Pearson Education, 2004
Wikimedia Foundation. 2010.
Look at other dictionaries:
Receiver (radio) — This article is about a radio receiver, for other uses see Radio (disambiguation). A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals … Wikipedia
Radio communication system — A radio communication system send signals by radio. [cite book | title = Wireless Network Performance Handbook | author = Clint Smith, Curt Gervelis | year = 2003 | publisher = McGraw Hill Professional | isbn = 0071406557 | url =… … Wikipedia
Radio receiver design — is a complex topic which can be broken down into a series of smaller topics. A radio communication system requires two tuned circuits each at the transmitter and receiver, all four tuned to the same frequency. [Cheney, M., Uth, R., Glenn, J.… … Wikipedia
Radio — is the transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light.Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of… … Wikipedia
radio — /ray dee oh /, n., pl. radios, adj., v., radioed, radioing. n. 1. wireless telegraphy or telephony: speeches broadcast by radio. 2. an apparatus for receiving or transmitting radio broadcasts. 3. a message transmitted by radio. adj. 4. pertaining … Universalium
Radio repeater — A radio repeater is a combination of a radio receiver and a radio transmitter that receives a weak or low level signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. This… … Wikipedia
Radio-controlled model — A radio controlled model (or RC model) is a model that is steerable with the use of radio control. All types of vehicles imaginable have had RC systems installed in them, including cars, boats, planes, and even helicopters and scale railway… … Wikipedia
Radio-controlled car — HPI E Savage, Tamiya Wild Willy 2, Kyosho Mini Z Monster, and Team Losi Micro T Radio controlled (or R/C) cars are self powered model cars or trucks that can be controlled from a distance using a specialized transmitter. The term R/C has been… … Wikipedia
Frequency modulation — See also: Amplitude modulation In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its… … Wikipedia
Radio jamming — is the (usually deliberate) transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without checking that it is in use first,… … Wikipedia