Regenerative circuit
[4][5][6] The regenerative receiver was invented in 1912[7] and patented in 1914[8] by American electrical engineer Edwin Armstrong when he was an undergraduate at Columbia University.
[5]: p.190 [11] Regeneration (now called positive feedback) is still widely used in other areas of electronics, such as in oscillators, active filters, and bootstrapped amplifiers.
[11][5]: p.190 It was never widely used in general commercial receivers, but due to its small parts count it was used in specialized applications.
One widespread use during WWII was IFF transceivers, where single tuned circuit completed the entire electronics system.
It is still used in a few specialized low data rate applications,[11] such as garage door openers,[12] wireless networking devices,[11] walkie-talkies and toys.
[13][3] Because of the large amplification possible with regeneration, regenerative receivers often use only a single amplifying element (tube or transistor).
[21] The regenerative detector provides sensitivity and selectivity due to voltage amplification and the characteristics of a resonant circuit consisting of inductance and capacitance.
The type 36 screen-grid tube (obsolete since the mid-1930s) had a non-regenerative detection gain (audio frequency plate voltage divided by radio frequency input voltage) of only 9.2 at 7.2 MHz, but in a regenerative detector, had detection gain as high as 7,900 at critical regeneration (non-oscillating) and as high as 15,800 with regeneration just above critical.
[16] Intrinsically, there is little or no difference in the gain and stability available from vacuum tubes, JFETs, MOSFETs or bipolar junction transistors (BJTs).
[25] For the reception of CW radiotelegraphy (Morse code), the feedback is increased just to the point of oscillation.
[26] 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.
Early vacuum tubes had low gain and tended to oscillate at radio frequencies (RF).
Since the advent of the transistor in 1946, the low cost of active devices has removed most of the advantage of the circuit.
In addition, the Q of the detector tuned circuit components vary with frequency, requiring adjustment of the regeneration control.
[24] 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.
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.
The inventor of FM radio, Edwin Armstrong, filed US patent 1113149 in 1913 about regenerative circuit while he was a junior in college.
So this design, getting most gain out of one tube, filled the needs of the growing radio community and immediately thrived.
[34] 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 and in the top-secret proximity fuze.
[36] In the 1930s, the superheterodyne design began to gradually supplant the regenerative receiver, as tubes became far less expensive.
Later it was almost completely phased out of mass production, remaining only in hobby kits, and some special applications, like gate openers.
After each quenching, RF oscillation grows exponentially, starting from the tiny energy picked up by the antenna plus circuit noise.
Superregenerative detectors work well for AM and can also be used for wide-band signals such as FM, where they perform "slope detection".
Regenerative detectors work well for narrow-band signals, especially for CW and SSB which need a heterodyne oscillator or BFO.
It is easily possible to build superregen receivers which operate at microwatt power levels, in the 30 to 6,000 MHz range.
For many years, superregenerative circuits have been used for commercial products such as garage-door openers, radar detectors, microwatt RF data links, and very low cost walkie-talkies.
But quenching with overtones acts further as a heterodyne receiver mixing additional unneeded signals from those bands into the working frequency.
