Miscellaneous

Edwin Armstrong Superheterodyne Radio Development

 Edwin Armstrong Superheterodyne Radio Development


Armstrong's new regenerative radio did not gain instant success. Vacuum tube / thermionic valve technology was not sufficiently developed and widely used to enable his idea to take off.

However before it could gain significant acceptable, war broke out in Europe and this lead to Armstrong's superheterodyne radio invention.

Armstrong in Europe

As the USA joined the war in Europe in 1917, Armstrong joined the US Army Signal Corps as an officer and he was sent to Paris.

In Paris, Armstrong was assigned to develop a system that could detect and listen to previously inaudible enemy short wave radio communications.There were two main issues that were being encountered with receivers for various forms of radio communication:

  • Lack of gain: In order to pick up the weak enemy radio communications signals, it was necessary to employ higher levels of gain than were previously available. The valves or vacuum tunes of the time oscillated at frequencies above about 500 kHz because of the large anode grid capacitance levels, and this severely limited the levels of gain achievable.
  • Lack of selectivity : Radio receivers of the time used tuning at the RF frequency. If using several sections to a filter, this meant that tuning from one frequency to another required all the filters to be re-tuned. This was an issue because ganged variable capacitors were not widely used at this time.

Previous developments

Prior to Armstrong starting work on the development of his new radio technology, others had set in place some foundations.

The first of these was that a Canadian engineer named Fessenden investigated a system of beats - his patent was filed on 28 September 1901. Although Fessenden's system used the transmission of two signals to create an audio beat, it was the first to outline this idea.

As the technology was not available to move Fessenden's idea of beats or heterodynes forwards, it lay dormant for a number of years.

The next mention of the system of heterodynes occurs when the American Navy did some experiments with them and found reception much easier.

A further development occurred when British engineer H J Round invented his "Autodyne" towards the end of 1913. This radio receiver used a valve to generate an oscillation and superimpose them on the incoming signal. This effectively formed a one valve direct conversion radio receiver.

As the war started, a number of engineers were investigating methods of improving sensitivity. Amongst these were H J Round, M Latour, L Levy, and later Edwin Armstrong on the side of the allies and W Schottky for the Germans.

One of the first major ideas to surface came from French engineer, Lucien Levy. He was concerned only with the reduction of interference. His idea was to bring the frequency of the incoming signals down to a region where they could be more suitably tuned. A lower frequency would allow for greater levels of selectivity.

Levy claimed that the ulta-acoustical beats would be so far separated in frequency from atmospheric disturbances and many interfering stations that they would be easily selected and be clear from interference.

Armstrong's superheterodyne research work

At the time Armstrong and the other American forces arrived in Europe, they were not aware of some of the advances that had been made, including those such as Round's autodyne and Levy's receiver using beats.

Armstrong set to work investigating the difficulties being experienced with amplifiers. While doing this, it occurred to him that the problem could be solved if the incoming signals were mixed with a locally generated high frequency signal. However, instead of producing an audible beat note, an "ultra-acoustical" signal could be produced at a frequency where the signals could be more easily amplified. After further amplification and filtering, the signals could be detected to produce audio signals, which could then be amplified as audio signals as appropriate.

In addition to this the ultra-acoustical amplifier could have filters at a fixed frequency that would enable far more effective filtering to be achieved.

Experiments into the Armstrongs's superheterodyne radio receiver were carried out by the Division of Research and Inspection of Signals Corps of the American Expeditionary Force.

An eight valve superheterodyne receiver was constructed. It consisted of a first detector (mixer); heterodyne oscillator, three stages of intermediate amplification, second detector, and two stages of audio amplification.

The armistice was signed before Armstrong's idea could be properly used and deployed. However Armstrong applied for a patent covering his ideas on 30 December 1918.

Other contenders

Although Armstrong is rightly credited with the invention of the superheterodyne radio, others were also investigating he technology. W Schottky also came up with the idea, and filed for a patent six months before Armstrong. However Armstrong was the first to develop the idea and produce a working radio.

Superheterodyne radio utilised

The end of hostilities the need for his new type of receiver dwindled. The superheterodyne radio used many more valves than other forms of receiver. Valves of the time were expensive to buy. They were also expensive to run because they required a number of batteries to power the heaters and the high tension anode circuits. Mains power supplies could not be used initially because the idea of an indirectly heated cathode had not come about allowing the use of separate no DC supplies for the valves.

In addition to this, broadcasting was in its infancy and the small number of stations meant the need for the selectivity provided by his new set was not needed.

However with the introduction of the indirectly heated cathode, a reduction in the cost of valves and the invention of the ganged variable capacitor combined with an dramatic increase In the number of broadcasting stations, the superheterodyne receiver came into widespread use around the end of the 1920s and beginning of the 1930s.


Watch the video: oscilloscope views inside a superheterodyne shortwave radio receiver (December 2021).