Tuned radio frequency receiver
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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. Prevalent in the early 20th 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. All tuned stages of the radio must track and tune to the desired reception frequency. This is in contrast to the later superheterodyne receiver which must only tune a frequency converter stage to the desired reception frequency. Many homemade radios constructed by enthusiasts today are tuned radio receivers, and these can range from single stage to multi-stage receivers.
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.
A problem with the TRF receiver in the time of triode vacuum tubes was that interelectrode capacitance (the so-called Miller capacitance) can cause instability and oscillation. In 1922, Louis Alan Hazeltine invented the technique of neutralization which uses an additional winding on the output or input tuned circuit to introduce an opposing signal which can cancel that capacitance, when properly adjusted. This was used in the popular Neutrodyne series of TRF receivers. The later adoption of the tetrode vacuum tube eliminated the Miller capacitance and the need for this touchy circuitry.
How it works
A 3 stage TRF receiver includes an RF stage, a detector stage, and an audio stage:
This schematic diagram shows a typical TRF receiver. This particular example uses six triodes. It has two radio frequency amplifier stages, one grid-leak detector/amplifier and three class ‘A’ audio amplifier stages. Generally, two or three RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage. The detector recovers the audio frequency signal from the modulated RF signal, and the audio stage amplifies the information signal to a usable level. The final stage was often simply a grid-leak detector.
Disadvantages of TRF receiver
Terman (1943, p. 658) characterizes the TRF's disadvantages as "poor selectivity and low sensitivity in proportion to the number of tubes employed. They are accordingly practically obsolete." Selectivity requires narrow bandwidth, and narrow bandwidth at a high radio frequency implies high Q or many filter sections. In contrast a superheterodyne receiver can translate the incoming high radio frequency to a lower intermediate frequency where selectivity is easier to achieve.
An additional problem for the TRF receiver is tuning different frequencies. All the tuned circuits need to track to keep the narrow bandwidth tuning. Keeping several tuned circuits aligned is difficult. A superheterodyne receiver only needs to track the RF and LO stages; the onerous selectivity requirements are confined to the IF amplifier which is fixed-tuned.
Although a TRF receiver can not be engineered for a high degree of selectivity relative to its carrier frequency, there is no reason it cannot reach the same level of sensitivity as other designs. The 1930s era BC-AN-229/429 military receiver was a six-valve design covering 201 to 398 kHz and 2.5 to 7.7 MHz (requiring several sets of plug-in coils to cover those ranges). This equipment probably exemplifies the limit of T.R.F. performance. Although the receiver bandwidth does vary, as noted above, the sensitivity of the set was around 8 microvolts for 10 milliwatts of audio output, comparable to that of the famous AN/ARC-5 superhet receiver that superseded it.
Although the TRF design has been largely superseded by superheterodyne and other circuits, it was "resurrected" in 1972 in silicon as the ZN414 TRF radio integrated circuit from Ferranti, thus affording the design a new lease of life in hobbyist radio projects, kits and some commercial products.
- Terman, Frederick E. (1943), Radio Engineers' Handbook, McGraw-Hill
- Tomasi, Wayne (2004), Electronic Communications Systems: Fundamentals Through Advanced (5th ed.), Pearson Education
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