Tuned radio frequency receiver
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A radio receiver comprised of several tuned radio-frequency amplifiers followed by circuits to detect and amplify the audio signal. Used in the early 20-th century, it is 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.
An additional 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.
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.