Editing Superheterodyne receiver (section)

=== Regeneration ===

[[Morse code]] was widely used in the early days of radio because it was both easy to produce and easy to receive.  In contrast to voice broadcasts, the output of the amplifier didn't have to closely match the modulation of the original signal.  As a result, any number of simple amplification systems could be used. One method used an interesting side-effect of early [[triode]] amplifier tubes. If both the plate (anode) and grid were connected to resonant circuits tuned to the same frequency and the stage gain was much higher than [[unity (mathematics)|unity]], stray [[capacitive coupling]] between the grid and the plate would cause the amplifier to go into oscillation.

In 1913, [[Edwin Howard Armstrong]] described a receiver system that used this effect to produce audible Morse code output using a single triode. The output of the amplifier taken at the anode was connected back to the input through a "tickler", causing [[feedback]] that drove input signals well beyond unity. This caused the output to oscillate at a chosen frequency with great amplification. When the original signal cut off at the end of the dot or dash, the oscillation decayed and the sound disappeared after a short delay.

Armstrong referred to this concept as a [[regenerative receiver]], and it immediately became one of the most widely used systems of its era. Many radio systems of the 1920s were based on the regenerative principle, and it continued to be used in specialized roles into the 1940s, for instance in the [[IFF Mark II]].