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tuned circuit

Electrically conducting pathway containing both inductance and capacitance elements. When these elements are connected in series, the circuit presents low electrical impedance to alternating current of the same frequency as the resonance frequency of the circuit and high impedance to current of other frequencies. The circuit's resonance frequency is determined by the values of inductance and capacitance. When the circuit elements are connected in parallel, the impedance is high at the resonance frequency and low at other frequencies. With their ability to pass only certain frequencies, tuned circuits are important in, for example, radio and television receivers.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

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Tuned radio frequency receiver

A tuned radio frequency receiver (TRF receiver) is a radio receiver that 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 receiver invented 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 Hazeltine invented the neutrodyne circuit, 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 frequency when 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=X_L/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 selectivity of 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 gains are 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.