Editing Superheterodyne receiver (section)

== Precursors ==

=== Early radio ===

Early [[Morse code]] radio broadcasts were produced using an [[alternator]] connected to a [[spark gap transmitter|spark gap]]. The output signal was at a [[carrier frequency]] defined by the physical construction of the gap, modulated by the [[alternating current]] signal from the alternator. Since the output frequency of the alternator was generally in the audible range, this produces an audible [[amplitude modulated]] (AM) signal. Simple [[Detector (radio)|radio detectors]] filtered out the high-frequency carrier, leaving the modulation, which was passed on to the user's [[headphone]]s as an audible signal of dots and dashes.

In 1904, [[Ernst Alexanderson]] introduced the [[Alexanderson alternator]], a device that directly produced radio frequency output with higher power and much higher efficiency than the older spark gap systems. In contrast to the spark gap, however, the output from the alternator was a pure carrier wave at a selected frequency. When detected on existing receivers, the dots and dashes would normally be inaudible, or "supersonic". Due to the filtering effects of the receiver, these signals generally produced a click or thump, which were audible but made determining dots from dashes difficult.

In 1905, Canadian inventor [[Reginald Fessenden]] came up with the idea of using two Alexanderson alternators operating at closely spaced frequencies to broadcast two signals, instead of one. The receiver would then receive both signals, and as part of the detection process, only the [[Beat (acoustics)#Mathematics and physics of beat tones|beat frequency]] would exit the receiver. By selecting two carriers close enough that the beat frequency was audible, the resulting Morse code could once again be easily heard even in simple receivers. For instance, if the two alternators operated at frequencies 3&nbsp;kHz apart, the output in the headphones would be dots or dashes of 3&nbsp;kHz tone, making them easily audible.

Fessenden coined the term "[[heterodyne]]", meaning "generated by a difference" (in frequency), to describe this system. The word is derived from the Greek roots ''hetero-'' "different", and ''-dyne'' "power".

=== 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]].

=== Radio direction finding ===

There was one role where the regenerative system was not suitable, even for Morse code sources, and that was the task of [[radio direction finding]], RDF.

The regenerative system was highly non-linear, amplifying any signal above a certain threshold by a huge amount, sometimes so large it caused it to turn into a transmitter (which was the entire basis of the original [[Identification friend or foe#IFF_Mark_II_anchor|IFF system]]). In RDF, the strength of the signal is used to determine the location of the transmitter, so one requires [[linear amplification]] to allow the strength of the original signal, often very weak, to be accurately measured.

To address this need, RDF systems of the era used triodes operating below unity. To get a usable signal from such a system, tens or even hundreds of triodes had to be used, connected together anode-to-grid. These amplifiers drew enormous amounts of power and required a team of maintenance engineers to keep them running. Nevertheless, the strategic value of direction finding on weak signals was so high that the [[British Admiralty]] felt the high cost was justified.