A superregenerative receiver is a radio receiver that achieves high sensitivity by alternately allowing a resonant circuit to oscillate and then suppressing the oscillation. Edwin Armstrong patented the technique in 1922 as an extension of the regenerative receiver. In each cycle, the amplification of a weak received signal grows rapidly and then decays when the gain of the circuitry is reduced. This can yield high sensitivity from simple, low-power circuitry. By the 1930s the technique was understood well enough for practical use and was used widely during the Second World War. Superregenerative receivers formed the receiving element in identification friend or foe (IFF) systems used to identify friendly aircraft and ships, and in beacon systems such as Rebecca–Eureka that helped aircraft locate ground positions. Large wartime production showed that superregenerative receiver designs could be made stable and reproducible despite earlier concerns about reliability. After the war, designers adopted superregenerative circuits for low-cost and battery-powered applications including hobby radio control systems, garage door openers, and wireless doorbells. Although more complex radio receiver designs later dominated communication systems, superregenerative techniques continued to be studied and used in specialized short-range applications, including updated theoretical analyses and millimeter-wave implementations.
History
Origin and early development The superregenerative receiver was patented in 1922 by Edwin Armstrong as an extension of the regenerative receiver. In that paper, Armstrong described a method in which a regenerative detector was periodically driven into and out of oscillation by a quench signal operating at a much lower frequency than the received radio signal. This produced repeated cycles of oscillation growth and decay. Because the amplification exceeded what had previously been considered the theoretical limit of regenerative amplification, Armstrong referred to the process as "super-regeneration". In his 1943 Edison Medal address, Armstrong later described the effect as arising from an unexpected observation during experimental work. At the time, regenerative receivers using a single tube typically provided gains on the order of a thousand, while more advanced receivers such as the superheterodyne achieved several thousand through multiple stages. In contrast, Armstrong reported observing gains as much as 100,000 in a single stage, which was not anticipated. He noted that the underlying principle was only understood after the phenomenon had been reproduced and studied, and later remarked that "a little work brought to light a principle quite beyond the bounds of one's wildest dreams". In 1922 Armstrong sold the rights for the superregenerative circuit to the Radio Corporation of America (RCA) for $200,000 in cash and 60,000 shares of RCA stock. This yielded more income than his earlier inventions. Lessing describes the price paid by RCA as reflecting expectations that the technique would have wide commercial use. In practice, superregenerative receivers proved poorly suited to broadcast reception as radio broadcasting evolved and stations became more closely spaced in frequency, since the technique lacked the selectivity needed to separate nearby signals. More selective receiver designs, notably the superheterodyne, were better suited to these conditions. RCA had initially placed high hopes for superregeneration, but David Sarnoff, then a vice president of the company, supported development of the superheterodyne. It later solved RCA's reception problem. Further theoretical analysis appeared during the 1930s. In 1938, F. W. Frink published a detailed treatment in the Proceedings of the IRE that described the difference between linear and logarithmic modes of operation and compared analytical results with laboratory measurements.
Superregenerative and regenerative techniques were also explored for portable radio use. A 1936 article in Wireless Engineer described a 20-pound portable duplex radiotelephone using superregenerative circuitry that functioned as both a receiver and a transmitter. The system reportedly operated in full duplex over short ranges, meaning that it transmitted and received at the same time. Oscillators at each end were synchronized in quench timing. When one unit was receiving, the other was transmitting. The two then exchanged roles at the quench rate.
Wartime applications Superregenerative receivers were used extensively during the Second World War, particularly in identification friend or foe (IFF) systems. They were employed in IFF Mark III airborne systems used by Allied forces. More than 200,000 such units were produced in the United Kingdom and the United States, with gain variation across units reportedly within 5 Decibels (dB) above or below reference values over a 30 Megahertz (MHz) band. Superregenerative receivers formed the "Eureka" portion of the Rebecca–Eureka radar navigation system. In this and related systems, a ground beacon responded to radar interrogation pulses with an active radio reply, allowing aircraft to locate the Eureka transmitter on the ground. These systems assisted aircraft operations during the Second World War. Large wartime production showed that superregenerative receivers could be made stable and reproducible, addressing earlier concerns about inconsistency.
Postwar consumer and hobby use After the war, superregenerative receivers became widely used in low-cost consumer and hobby applications. A June 1947 issue of Electronics magazine described a single-tube superregenerative receiver using a thyratron for hobby radio-control systems. Raytheon also published a circuit design combining a tube and a transistor. The simplicity and high sensitivity of the design made the receiver well-suited for inexpensive remote-control equipment. Superregenerative receivers were later adopted in short-range consumer devices such as garage door openers, wireless doorbells, and radio-controlled toys. Their low component count, low power consumption, and sufficient performance for simple short-range radio links contributed to their use in these products.
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