History of Radio and Electronics
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- Note on the Superhet Radio History
- Superhet Radio History
- Superhet gains acceptance
- Further refinements
The superhet radioWith the rise in use of wireless technology in a variety of areas, the need for greater selectivity became more apparent. Crystal sets had neither the gain nor the selectivity needed, and also the new tuned radio frequency, TRF sets lacked the levels of selectivity required, especially as the number of transmitting stations rose and there was a need for greater levels of performance. To combat this, the superheterodyne radio or to give it its shorter name the superhet radio rose in popularity, especially as the number of broadcast stations rose.
- history of the development of the superhet or superheterodyne radio receiver from its beginnings with the discover of beats to the double and triple superhets for high performance.To solve the problem a new type of receiver known as the superhet, or supersonic heterodyne receiver was developed. The first step in this development was taken by a Frenchman named Lucien Levy. He was investigating ways in which receiver selectivity could be improved and in doing this he devised a system whereby the signals were converted down to a lower frequency where the filter bandwidths could be made narrower. A further advantage was that the gain of valves was considerably greater at the lower frequencies after used after the frequency conversion, and there were fewer problems with the circuits bursting into oscillation. The idea was very successful, and even though it did not totally eliminate interference as Levy had hoped it was a considerable improvement over previous receivers. However it was not the superhet of today because his receiver still retained the idea of a variable frequency filter, even though it was at a lower intermediate frequency. The idea for developing a receiver with a fixed intermediate frequency amplifier and filter is credited to Edwin Armstrong. Working for the American Expeditionary Force in Europe, Armstrong thought that if the incoming signals were mixed with a variable frequency oscillator, a low frequency fix tuned amplifier could be used. Like Levy's idea this would enable the valves to operate at a lower frequency where they would be more efficient. It also meant that a fix tuned amplifier could be used and this would be capable of providing much greater degrees of selectivity than a variable one. This is because several stages could be cascaded relatively easily and the tuning preset before use. Armstrong's original receiver consisted of a total of eight valves. In the set the signal was converted from its incoming frequency down to a fixed intermediate frequency stage. By altering the frequency of the local oscillator the frequency of the received signal was changed. The low intermediate frequency stage allowed greater levels of gain, as in the case of Levy's set because the low frequencies allowed greater levels of gain and stability. Also having a fixed frequency intermediate stage allowed the filters to be more selective. Several tuned circuits could be cascaded to improve selectivity, and being on a fixed frequency they did not all need to be changed in line with one another. The filters could be preset and left correctly tuned. The new superhet gave an impressive performance, but its development came at the end of the war. With its use of a large number of valves it was only viable for use in specialist applications, many of which were no longer required after the cessation of hostilities. Accordingly Armstrong's discovery was rarely used for a number of years. Interestingly, Armstrong was not the only person working on the idea of a superhet. Meissner in Germany took out a patent for the idea six months before Armstrong, but as Meissner did not prove the idea in practice and did not build a superhet radio, the idea is credited to Armstrong. Superhet gains acceptance It took many years before the superhet was widely used. In the early 1920s there were few stations that could be heard and the superior performance of the superhet was not required, especially at the considerable cost of all the valves that were used. However as the 1920s passed more stations came on the air, especially in America and the need for the selectivity provided by the superhet became more apparent. As a result the superhet started to be used. A number of developments in valve technology also helped. Originally all valves were directly heated, and this meant that each valve required a separate filament supply. The introduction of the indirectly heated valve meant that an alternating supply could be used. This was because the heaters different connection to the cathode. This allowed the bias conditions to be fulfilled even if the filaments were connected in series or parallel with one another.
 The use of superhets by domestic users forced further developments to be made. Originally these sets were very cumbersome and sometimes difficult to use. Ganged tuning capacitors were also introduced to enable the local oscillator and the radio frequency stages to be tuned by one control. Many other refinements were added enabling them to be made more cheaply and easier to use.
The need for the increased performance of the superhet was first felt in America and by the late 1920s most sets were superhets. However in Europe the number of broadcast stations did not start to rise as rapidly until later. Even so by the mid 1930s virtually all sets in Europe as well were using the superhet principle. Valve technology improved again with the introduction of further electrodes into the envelope. In 1926 the tetrode valve was introduced. This valve had a second grid placed between the normal control grid and the anode. This had the effect of reducing the capacitance causing the feedback, and enabled valves to operate far more reliably than they had done before. Three years later the tetrode was improved by the introduction of the pentode. In this type of valve, yet another grid called the suppressor grid was introduced. This overcame a discontinuity in the curve of the tetrode, and enabled further improvements in their performance.
Once the use of the superrhet radio was established it became virtually the only form or topology of radio that was used. Various forms were developed including double and even triple superhet radios and they had correspondingly high levels of performance.
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