Background
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.
Read more about this topic: Tuned Radio Frequency Receiver
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