![]() 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 IFF system). 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. 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. 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.Īrmstrong referred to this concept as a regenerative receiver, and it immediately became one of the most widely used systems of its era. This caused the output to oscillate at a chosen frequency with great amplification. ![]() 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. In 1913, Edwin Howard Armstrong described a receiver system that used this effect to produce audible Morse code output using a single triode. 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, stray capacitive coupling between the grid and the plate would cause the amplifier to go into oscillation. One method used an interesting side-effect of early triode amplifier tubes. As a result, any number of simple amplification systems could be used. In contrast to voice broadcasts, the output of the amplifier didn't have to closely match the modulation of the original signal. Morse code was widely used in the early days of radio because it was both easy to produce and easy to receive. The word is derived from the Greek roots hetero- "different", and -dyne "power". For instance, if the two alternators operated at frequencies 3 kHz apart, the output in the headphones would be dots or dashes of 3 kHz tone, making them easily audible.įessenden coined the term " heterodyne", meaning "generated by a difference" (in frequency), to describe this system. 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. The receiver would then receive both signals, and as part of the detection process, only the beat frequency would exit the receiver. 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. Due to the filtering effects of the receiver, these signals generally produced a click or thump, which were audible but made determining dot or dash difficult. When detected on existing receivers, the dots and dashes would normally be inaudible, or "supersonic". In contrast to the spark gap, however, the output from the alternator was a pure carrier wave at a selected frequency. ![]() 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. Simple radio detectors filtered out the high-frequency carrier, leaving the modulation, which was passed on to the user's headphones as an audible signal of dots and dashes. Since the output frequency of the alternator was generally in the audible range, this produces an audible amplitude modulated (AM) signal. 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. ![]() Virtually all modern radio receivers use the superheterodyne principle.Įarly Morse code radio broadcasts were produced using an alternator connected to a spark gap. It was long believed to have been invented by US engineer Edwin Armstrong, but after some controversy the earliest patent for the invention is now credited to French radio engineer and radio manufacturer Lucien Lévy. A 5-tube superheterodyne receiver made in Japan circa 1955 Superheterodyne transistor radio circuit circa 1975Ī superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. ![]()
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