  thumb|upright=1.2|tuning a trf receiver, like this 5 tube neutrodyne set from 1924 with two stages of rf amplification, was a complicated process. the 3 tuned circuits, controlled by the 3 large knobs, had to be tuned in unison to the new station. so tuning in a station was a process of successive approximation. once a station was found, the numbers on the dials were written down, so it could be found again.   a tuned radio frequency receiver (or trf receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (rf) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an audio frequency amplifier. popular in the 1920s, it could be tedious to operate because each stage must be individually tuned to the station's frequency. by the mid 1930s it was replaced by the superheterodyne receiver invented by edwin armstrong. == 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. multiple stages of rf amplification would make the radio more sensitive to weak stations, and the multiple tuned circuits would give it a narrower bandwidth and more selectivity than the single stage receivers common at that time. all tuned stages of the radio must track and tune to the desired reception frequency. this is in contrast to the modern superheterodyne receiver that must only tune the receiver's rf front end and local oscillator to the desired frequencies; all the following stages work at a fixed frequency and do not depend on the desired reception frequency. 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 built with triode vacuum tubes was the triode's interelectrode capacitance. the interelectrode capacitance allowed energy in the output circuit to feedback into the input. that feedback could cause instabili
History_of_radio Radio_electronics Receiver_(radio) Wireless_tuning_and_filtering