I have been reading about radio receivers and found about the regenerative and the superregenerative designs, both from Edwin Armstrong. But I can't find anywhere a good explanation about the difference between how they work.
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Regenerative receiver uses deliberate positive feedback (called "regeneration") to increase the gain of the RF amplifier. A useful side-effect is to sharpen the tuning (useful search term : Q multiplier).
This regeneration is adjustable; the art of tuning a regenerative receiver for a weak station is to get the positive feedback as high as possible without allowing oscillation to start.
As the BBC Handbook (1928) says...
The super-regen, on the other hand, allows the oscillation to start, but "quenches" it at a frequency above the desired channel bandwidth (above the 5kHz or so audio bandwidth of an AM radio.)
Received RF energy helps the oscillation to start earlier, and build up faster to a high amplitude before the quenching happens, so the effect is to give a super-regenerative receiver enormously high gain and sensitivity.
However it cannot distinguish between tiny signals on the tuned frequency or larger ones slightly off station, so this sensitivity comes at some cost in interference rejection.
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\$\begingroup\$ As well as on-frequency RF regeneration, tube radios could also then feed the audio back through the RF tube, as it can amplify two totally separate frequencies without interference. Not sure how common this was, but in an era where tubes used batteries for power, reduced tube count=reduced running cost. \$\endgroup\$ Commented Jul 14, 2018 at 22:58
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\$\begingroup\$ @Henry That technique is called 'reflex'. It doesn't have anything to with regeneration. \$\endgroup\$ Commented Jul 14, 2018 at 23:44
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\$\begingroup\$ The drawback of super-regen is that because they oscillate, they radiate back out the antenna quite strongly (and over quite a broad bandwidth). That has seen them out of favour nowadays. During a war, it makes them easy to track down. Don't know if they found that out the hard way, or knew not to try it in the first place. \$\endgroup\$ Commented Jul 15, 2018 at 1:37
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\$\begingroup\$ @EJP Never knew the name for it, but recall seeing it used in regen receivers. I wonder if they were used together in the early days, before the advent of the germanium diode, as it seems to require a separate detector device. \$\endgroup\$ Commented Jul 15, 2018 at 2:27
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1\$\begingroup\$ @HenryCrun you are right, most reflex receivers also used regeneration (not superregen) though they are separate concepts. Superregen is also not terribly linear, so it's not ideal for AM, it's more often used for CW (Morse or nowadays on/off keying) or other modulation systems that can tolerate poor linearity. And you're also right that oscillating (in-band) does tend to re-radiate, which is why "the good listener does not oscillate" - all your neighbours can hear your receiver squealing! \$\endgroup\$– user16324Commented Jul 15, 2018 at 11:02
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The superregenerative detector is well described by Brian Drummond. A few words may be said about regenerative detectors.
A regenerative detector may be set to three modes of operation.
First the regular AM detector. The regeneration is set to be under self oscillation. The resonant circuit uses the incoming signal to oscillate. All losses are not compensated. If all losses were compensated the resonant circuit would oscillate at a constant level rendering AM detection impossible.
In theory this first mode of operation can also detect FM.
Second mode is the regeneration setting to just compensate the losses as described above. It is possible to design regenerative detectors with very smooth regeneration management. The onset of this borderline mode may be observed as a sudden loss of audio when the regeneration is increased.
The third mode is when the regeneration is set just a little over full compensation and the resonant circuit oscillates weakly on its own. This mode can detect FM when the FM signal is tuned to either slope of the resonant circuit resonance curve.
When the signal is introduced it tries to pull or push the oscillation to the signal freqency. The detector "fights back" by increasing the current consumption. The more the incoming FM deviates from the resonce circuit natural resonance, the more current the detector consumes. These current variations may be amplified yielding FM detection.
In theory this third mode can also detect AM when the incoming carrier is set to the resonance circuit slope. It is likely that the AM detection is not as good as FM detection because the FM carrier delivers a much stronger push-pull force on the resonant circuit.
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\$\begingroup\$ CW and ssb can be received when the regen is turned up to oscillation (mode 3) youtube.com/watch?v=Fij81JjZ8r0 \$\endgroup\$ Commented Jul 15, 2018 at 2:23