Superheterodyne receiver

The superheterodyne receiver, commonly called the superhet, is a radio receiver that uses heterodyning to convert incoming radio-frequency (RF) signals to a fixed intermediate frequency (IF). The signal is then amplified and filtered at that fixed frequency. This arrangement separates tuning from most of the gain and filtering: the RF circuits make an initial, relatively broad selection of the desired station, while the IF stages provide most of the amplification and the selectivity needed to separate it from adjacent stations. The design became important during the rapid growth of broadcast radio in the 1920s. As amplitude modulation (AM) stations multiplied, receivers had to handle crowded bands and signals that ranged from strong local stations to weak distant ones. Many earlier sets required several tuning controls to be adjusted together, and their performance varied widely. The superheterodyne offered a more practical path to stable gain, sharper selectivity, and simpler operation, especially as vacuum tubes (valves) improved and became cheaper. The principle had been developed earlier, but the superheterodyne did not become widely used until the mid-1920s, when receiver designs and vacuum tubes improved enough for practical mass production. Patent control and licensing also played a role. The Radio Corporation of America (RCA) and associated companies held key rights and influenced which receiver types could be manufactured. By the early 1930s, as licensing issues eased, the superheterodyne largely replaced earlier receivers.

History

Radio direction finding Radio direction-finding (RDF) required the accurate amplification of weak signals. RDF equipment used in World War I operated at frequencies from about 50 kHz to 2 MHz. Armstrong stated that the motivation for the superheterodyne was to extend the usable frequency range of such systems, allowing reception of higher-frequency emissions, such as those from aircraft ignition.

Origins Incoming radio signals are very weak, and early sets were limited by the available means of amplification. Crystal detector receivers provided rectification without gain and were widely used in inexpensive sets produced by companies such as Crosley. The introduction of the audion (triode) made amplification possible and enabled louder reception, but early vacuum tubes were expensive and had limited performance. Much of early receiver architecture was driven by the cost of gain; in 1920 a triode cost $7.00, about $113 now. Receiver performance is commonly described in terms of sensitivity, selectivity, fidelity, distortion, and noise. Sensitivity is the ability to receive weak signals at a usable level, while selectivity is the ability to discriminate the desired signal from signals at other frequencies. Fidelity and distortion describe the accuracy of the recovered audio, while receiver noise limits the weakest signal that can be used. Herold placed the years from about 1907 to 1927 in the first major period of receiving-tube development. The triode gave receiver designers a practical amplifier, and it also made oscillators, feedback circuits, heterodyne reception, and the superheterodyne possible. Since tubes were still expensive and limited, many receiver circuits of the period were attempts to get useful gain and selectivity from as few tubes as possible. Several approaches followed:

Regenerative receivers increased gain and selectivity with feedback. Reflex receivers reused a single tube as two amplification stages to reduce cost. The Neutrodyne stabilized tuned radio-frequency amplifiers by canceling unwanted feedback, allowing more gain per tube. Superregenerative receivers, which achieve very high gain, were developed by Edwin H. Armstrong in 1922 while studying the regenerative receiver. The circuit did not become widely used in broadcast or television reception. Broadcasting expanded rapidly, increasing the number of stations and available programs. Westinghouse entered broadcasting after Frank Conrad began transmitting from his home soon after World War I. The number of U.S. broadcasting stations grew from five in 1921 to 530 in 1924. By the late 1920s, the growth of broadcasting placed greater demands on receivers. As one contemporary review observed, “receivers which were giving satisfactory service at the beginning of that period are now obsolete.” With more stations on the air, sets needed better selectivity. Listeners also wanted to hear weaker distant stations, and improved loudspeakers made poor audio more noticeable. At the same time, vacuum tubes improved and became less expensive. By 1925, the price of a triode had fallen to $3.59, and by 1936 to $0.59. As tubes became cheaper, the extra stages required by the superheterodyne became less of an economic disadvantage. Improved tubes also made stable gain, single-control tuning, and automatic gain control more practical, helping the superheterodyne become the standard broadcast receiver architecture.

… excerpt ends here. Continue reading the full article.