, in which the feedback was applied to the input (grid) of the tube with a "tickler coil" winding on the tuning inductor. In a regenerative receiver, a portion of the detector's RF output is fed back to its input through a tuned circuit, providing frequency-selective positive feedback. When adjusted below oscillation, this feedback substantially increases sensitivity and selectivity, allowing RF amplification and detection to be implemented using a single active device. Regeneration sharpens the receiver's frequency response by increasing loop gain near resonance. The intrinsic Q of the tuned circuit itself is unchanged; instead, feedback compensates for circuit losses, producing behavior mathematically equivalent to reducing resistive loss. As the loop gain approaches unity, the effective bandwidth narrows rapidly. Oscillation begins when losses are fully compensated. Providing the oscillation separately from the detector allows the regenerative detector to be set for maximum gain and selectivity - which is always in the non-oscillating condition.
CW reception (autodyne mode) For the reception of
CW radiotelegraphy (
Morse code), the feedback is increased just to the point of oscillation. The tuned circuit is adjusted to provide typically 400 to 1000 Hertz difference between the receiver oscillation frequency and the desired transmitting station's signal frequency. The two frequencies
beat in the nonlinear amplifier, generating
heterodyne or
beat frequencies. The difference frequency, typically 400 to 1000 Hertz, is in the audio range; so it is heard as a tone in the receiver's speaker whenever the station's signal is present. Demodulation of a signal in this manner, by use of a single amplifying device as oscillator and
mixer simultaneously, is known as
autodyne reception. The term
autodyne predates multigrid tubes and is not applied to use of tubes specifically designed for frequency conversion.
SSB reception For the reception of
single-sideband (SSB) signals, the circuit is also adjusted to oscillate as in CW reception. The tuning is adjusted until the demodulated voice is intelligible.
Advantages and disadvantages Regenerative receivers require fewer components than other types of receiver circuit, such as the
TRF and
superheterodyne. The circuit's advantage was that it got much more amplification (gain) out of the expensive
vacuum tubes, thus reducing the number of tubes required and therefore the cost of a receiver. Early vacuum tubes had low gain and tended to oscillate at
radio frequencies (RF). TRF receivers often required 5 or 6 tubes; each stage requiring tuning and neutralization, making the receiver cumbersome, power hungry, and hard to adjust. A regenerative receiver, by contrast, could often provide adequate reception with the use of only one tube. In the 1930s the regenerative receiver was replaced by the superheterodyne circuit in commercial receivers due to the superheterodyne's superior performance and the falling cost of tubes. Since the advent of the
transistor in 1946, the low cost of active devices has removed most of the advantage of the circuit. However, in recent years the regenerative circuit has seen a modest comeback in receivers for low cost
digital radio applications such as
garage door openers,
keyless locks,
RFID readers and some
cell phone receivers. A disadvantage of this receiver, especially in designs that couple the detector tuned circuit to the antenna, is that the regeneration (feedback) level must be adjusted when the receiver is tuned to a different frequency. The antenna impedance varies with frequency, changing the loading of the input tuned circuit by the antenna, requiring the regeneration to be adjusted. In addition, the Q of the detector tuned circuit components vary with frequency, requiring adjustment of the regeneration control. Another drawback is that when the circuit is adjusted to oscillate it can radiate a signal from its antenna, so it can cause
interference to other nearby receivers. Adding an RF amplifier stage between the antenna and the regenerative detector can reduce unwanted radiation, but would add expense and complexity. Other shortcomings of regenerative receivers are the sensitive and unstable tuning. These problems have the same cause: a regenerative receiver's gain is greatest when it operates on the verge of oscillation, and in that condition, the circuit behaves
chaotically. Simple regenerative receivers electrically couple the antenna to the detector tuned circuit, resulting in the electrical characteristics of the antenna influencing the resonant frequency of the detector tuned circuit. Any movement of the antenna or large objects near the antenna can change the tuning of the detector.
History For the broader role of patents and licensing in early radio development, see The inventor of
FM radio,
Edwin Armstrong, filed US patent 1113149 in 1913 about regenerative circuit while he was a junior in college. He patented the superregenerative circuit in 1922, and the
superheterodyne receiver in 1918.
Lee De Forest filed US patent 1170881 in 1914 that became the cause of a contentious lawsuit with Armstrong, whose patent for the regenerative circuit had been issued in 1914. The lawsuit lasted until 1934, winding its way through the appeals process and ending up at the
Supreme Court. Armstrong won the first case, lost the second, stalemated at the third, and then lost the final round at the Supreme Court. At the time the regenerative receiver was introduced,
vacuum tubes were expensive and consumed much power, with the added expense and encumbrance of heavy batteries. So this design, getting most gain out of one tube, filled the needs of the growing radio community and immediately thrived. Although the superheterodyne receiver is widely used today, the regenerative radio made the most out of very few parts. In World War II the regenerative circuit was used in some military equipment. An example is the German field radio "Torn.E.b". Regenerative receivers needed far fewer tubes and less power consumption for nearly equivalent performance. A related circuit, the
superregenerative detector, found several highly important military uses in World War II in
Friend or Foe identification equipment]. An example here is the miniature RK61
thyratron marketed in 1938, which was designed specifically to operate like a
vacuum triode below its ignition voltage, allowing it to amplify analog signals as a self-quenching superregenerative detector in
radio control receivers, and was the major technical development which led to the wartime development of radio-controlled weapons and the parallel development of
radio controlled modelling as a hobby. In the 1930s, the
superheterodyne design began to gradually supplant the regenerative receiver, as tubes became far less expensive. In Germany the design was still used in the millions of mass-produced German "peoples receivers" (
Volksempfänger) and "German small receivers" (DKE, Deutscher Kleinempfänger). Even after WWII, the regenerative design was still present in early after-war German minimal designs along the lines of the "peoples receivers" and "small receivers", dictated by lack of materials. Frequently German military tubes like the "RV12P2000" were employed in such designs. There were even superheterodyne designs, which used the regenerative receiver as a combined IF and demodulator with fixed regeneration. The superregenerative design was also present in early FM broadcast receivers around 1950. Later it was almost completely phased out of mass production, remaining only in hobby kits, and some special applications, like gate openers. ==Superregenerative receiver==