by PETER LANKSHEAR

Who really DID invent the Superheterodyne?

The invention of the superheterodyne is one of the most important developments in the history of radio, and is basic to the operation of virtually every receiver and many transmitters in operation today. Without the `superhet', modern electronic communications would not be possible. Yet for all its significance, its exact origins have been controversial.

The basic principle of the super- correspondence in EA on the subject, igation (more than once involving Lee heterodyne is simply that any two sig and the 'When I Look Back' column de Forest). Depending on the source of nals can, when mixed together, create for July 1990, which described the writing, significant developments are other signals that have a mathematical accomplishments and tragedy of attributed to different inventors and relationship to the original pair. For Edwin Armstrong, put into perspective piecing together the true story can be a centuries musicians have known of the part played by the Frenchman time consuming but fascinating task. this effect, called beating, in which Lucien Levy in the invention of the Radio is of course not the only two notes will appear to the ear to pro- superhet. field where this has occurred. Morse duce a third. In the superhet, the equiv- At about that time, Winston Muscio and the electric telegraph, Edison and alent beat note is, of course, the inter- (a retired STC engineer) provided me the electric lamp and the phonograph, mediate frequency. with some interesting data, including a Bell and the telephone have all Although radio matured during the copy of the Levy patents, which achieved immortality for their inven- 1920s, with the TRF the dominant made the events a little clearer to me. tiveness, but in each case their work type, the superhet is the older technol- However, as we shall see, recently sig- was closely paralleled by that of ogy, having been around for the best nificant information about the French other researchers. part of 80 years. But who invented it? involvement has become available and Many Americans give the credit to clearly throws some doubt on Levy's Fessenden's heterodyne Edwin Armstrong, but others recog- integrity. Marconi's transmitters, based on nise the work of Lucien Levy. Early radio was often marred with the experiments of Hertz, used high A few years ago, there was some patent claims and sometimes bitter lit- voltage discharges across spark gaps to excite tuned circuits and aerials into oscillation and thereby generate RF signals — which were, in effect, modulated by the individ- ual sparks. Progressive engineers, among them the Canadian Professor Reginald Fessenden, realised that spark transmissions were inefficient and crude and that real progress lay in using continu- ous waves; but he realised that existing detectors would produce very little audio output from an unmodulated signal. A further problem was lack of sensi- tivity, in the coherers and magnetic detectors of the period. Fessenden had observed that by far the most sensitive One of the world's finest, largest and longest established collection of vintage instrument available was the standard radio equipment is at the Antique Wireless Association's Museum in the telephone earpiece — but of course charming little New England style village of East Bloomfield, in New York this could not be used to receive radio State. Among their many priceless and historically significant acquisitions are signals directly. several pieces of equipment constructed by Edwin Armstrong. This is a resis- His solution, patented in 1902, was tance coupled IF amplifier strip, built to demonstrate his superheterodyne the simultaneous transmission of two receiver. The device with the label attached is the filament rheostat! signals with a small difference in fre- 92 ELECTRONICS Australia, December 1995 quency. These supersonic signals were Regardless of the true inventor, the each fed to individual windings of a oscillator enabled the heterodyne special headphone and an audible dif- receiver to become a practical device, ference beat was registered by the and was widely used by various navies operator's ear. Thus, if the two signals during the first World War. were separated by a difference of lkHz, a note of that frequency would WW1 spurred development be heard in the earpiece. The period 1914-18 saw consider- Fessenden also realised that only able radio development on both one transmitter would be necessary if sides. From an erratic curiosity the second signal was generated by a which Lee de Forest, the nominal local oscillator. This was the origin of inventor, did not fully understand, the beat frequency oscillator or `13F0', the triode valve was made into a sta- and for the note produced, Fessenden ble and practical device. gave us the term 'heterodyne' from the By 1917, when America entered Greek meaning 'other force'. the War, radio was an integral part of Heterodyne reception had the advan- warfare, and Armstrong, by now an tage too of improving detector effi- acknowledged authority on reception, ciency, by in effect adding energy to was soon given a commission and the audible signal. sent to France to research military radio applications. Ahead of its time On his way to France, in October Valves did not exist in 1902, and it 1917, Armstrong had been able to visit was not until after the invention of This diagram of a receiver using London, and called on Captain Henry the valve oscillator a decade later Fessenden's heterodyne principle is Round of\ the Marconi Company. The that the heterodyne detector came taken from the 1925 Admiralty two 'hit it off' immediately, and Round Handbook. Before the advent of the into its own. Until then the local sig- valve, the source of heterodyne oscil- was able to demonstrate his 14-stage nal had to be generated by a minia- lations was an arc generator or a high RF amplifier, which had been used for ture arc transmitter, or even a high frequency alternator. direction finding by eavesdropping on frequency alternator! Obviously the German Fleet's low powered inter- these were unwieldy methods, and a fleet communications while they were classic example of a system being in As an indication of the closeness of stationed in Heligoland. The Royal advance of technology. the race, in 1913 Meissner took out his Navy was alerted to movements of the The invention of the valve oscillator patent on April, Franklin's patent deal- German ships in harbour, and the out- was a landmark in radio history, and a ing primarily with regeneration was come was the Battle of Jutland. classic example of simultaneous effort registered in June and Armstrong's Round's amplifier illustrates the dif- in different countries. In September patents were filed in October. ficulties of the time in obtaining useful 1912, whilst still a student at New Armstrong was an impecunious stu- RF amplification. Although he devel- York's Columbia University, Edwin dent who, with more resources, could oped a special low capacitance valve, Armstrong demonstrated the regenera- have applied for his patent earlier. the baseless V24, he could only tive detector and the closely related History has credited Armstrong as achieve a stage gain of two. However, valve oscillator. Meanwhile, in being first to make a valve oscillate in given a sufficient number of stages, Germany the Telefunken engineer his discovery of the regenerative detec- sufficient amplification to reach to the Alexander Meissner and in England, tor, although De Forest eventually won noise level of the aerial was possible. Marconi engineers Franklin and the legal battle on the dubious strength By all accounts, the Round receiver Round had been successful in produc- of getting audio feedback between a was extremely difficult and time con- ing valve oscillators. microphone and telephone earpiece. suming to set up and tune. For practical

Diagram 2 (left): Levy's patent application included this configuration in which an IF amplifier was optional. His prime claim was for interference cancellation, with the superheterodyne principle almost as an afterthought. Diagram 3 (right): Armstrong's ideas were much clearer than Levy's, and this diagram from his superheterodyne patent application shows a practical and recognisable configuration using an IF amplifier labelled A. ELECTRONICS Australia, December 1995 93 VINTAGE RADIO

Diagram 4 (left): Schottky's proposed receiver was essentially identical to Armstrong's, a remarkable example of how the same invention can originate at the same time from two completely independent sources. Diagram 5 (right): The dia- gram that accompanied Laut's February 1, 1917, report to Ferrie suggesting the use of supersonic heterodyne signals. This is a practical circuit, although some tuning ahead of the resistance coupled IF amplifier would be of considerable benefit.

work, simpler methods of amplifica- The French connection superheterodyne has been questioned. tion were needed. It is significant that the French The next part of the story has During World War One, the French. Government recognised Armstrong's become part of the folk lore of early set up a radio research organisation patent, and according to his biograph- radio. At this time, most radio commu- under Colonel Ferrie, one member of er, presented him with a medal. nication was carried out at frequencies the team being engineer Lucien Levy. However, in another of Armstrong's below 1MHz, but there was a belief Levy's chief interest seems to have disappointments, Levy later claimed that the Germans were using much been in noise cancellation, a common that Armstrong had stolen the superhet higher frequencies where valves could enough research subject in the days of concept from him. Several years later, not be made to amplify. low frequency transmissions with their in an implied recognition, American Sensitive as it is, the regenerative heavy static background. In August Telephone and Telegraph bought detector has a lower limit to signal 1917, he made a long and somewhat Levy's US patent application for strengths to which it will respond, and rambling patent application dealing $20,000. Of the nine claims in the only remedy is pre-detection ampli- with problems in radio transmissions: Armstrong's 1919 patent, all were lost fication, which was not possible with `atmospheric disturbances, confusions in later interference proceedings in the valves of the type then used by the mil- between the different emcissions, and US Patent office. itary. Armstrong gave thought to this the lack of secrecy'. We will never know whether or not impasse, and is said to have thought of Finally, after several obscure pages Armstrong was influenced by ideas the solution while watching a night describing what seems to be interfer- from Levy, or developed his invention time air raid on Paris. ence reduction, and almost as an after- completely independently. But it is It occurred to him that it might be thought, he mentions what has been certain that there was no contact with possible to locate aircraft by track- identified as the superheterodyne prin- W. Schottky of the Siemens laboratory ing the high frequency radiation ciple stating: in Germany, who applied on the 18th from their ignition systems, but A method as claimed in Claim 1 June 1918 for a patent that was almost considerable amplification would permitting of the selective reception identical to the application that be necessary. of the ordinary radio-telegraphic or Armstrong made six months later. To obtain the necessary amplifica- radio-telephonic sustained-wave Clearly, there could have been no com- tion, he proposed to use a receiver emissions, characterised by the fact munication between Armstrong and with a heterodyne detector to produce that ultra-acoustic beats are produced Schottky; so it is apparent that here a supersonic rather than audible beat by the combination at the receiving was a classic case of simultaneous signal — which could be amplified, station of the current coming from the invention. However Schottky's work using existing technology, to a level transmitting station with the current was purely theoretical — he did not suitable for conventional detection. from a first local generator of high build a working example. His name for the new system was the frequency current, followed in the To summarise, the generally accept- super(sonic)heterodyne. case of radio-telegraphic reception, ed situation has been that Levy, Armstrong developed and just as the by the production of beats of acousti- Schottky and Armstrong were all war ended, demonstrated a working cal frequency, by the combination of granted patents for the superhetero- eight-valve 'super heterodyne' receiv- the ultra-acoustical beats with a sec- dyne receiver. Levy was first, but his er. The first patent was filed in Paris on ond oscillating local generator of ideas were somewhat obscure, concen- December 30, 1918. His American ultra-acoustical frequency. trating more on interference reduction, patent, which even allowed for multi- When compared with Armstrong's with the supersonic heterodyne princi- ple frequency conversion, was filed on precise wording and his working ple almost an afterthought. Schottky's February 8th 1919, and was issued on receiver, it is understandable that patent application was next, with a June 8, 1920. Levy's claim to have invented the clear description of a workable system, 94 ELECTRONICS Australia, December 1995 but his work was theoretical only. new TM valves. Laut's specialist and one protagonist was unsporting Finally, Armstrong delayed his patent research subject was heterodyne recep- enough to introduce the subject of Paul application until he had a working tion. As we have already seen, another Laut's report on the heterodyne receiv- receiver with advanced and detailed member of the team was Lucien Levy. er to Colonel Ferrie. concepts of its potential. In 1916, Laut was hospitalised with Following some more acrimony in Levy may have been first to the tuberculosis, but was able to continue which Levy tried to play down the patent office, but Armstrong was con- his theoretical work on heterodyne event, the Editor of L'Antenne waded sidered by his peers to be the father of reception. In the process, it occurred in with a transcript of the Laut report the superheterodyne. Given the nature to him that the beat note, although at which had been kept and made avail- of the man, and his ability, proven by an inaudible frequency, could still be able by one of the Eiffel Tower team. his later achievements in developing readily amplified. On February 1st, Touché Monsieur Levy! super-regeneration and pioneering FM 1917, Laut presented a report on this As Winston Muscio commented transmissions, it is highly unlikely that to Colonel Ferrie — who, as was nor- after reading the Champeix article, he needed to or would have stooped to mal practice, distributed it to other "When everyone gets their just deserts stealing Levy's ideas, as the latter at team members. in the hereafter, it will be Laut who one time suggested. Later in 1917, Laut returned to the gets the superhet medal". ❖ team, but was surprised and upset to Twist in the tale find that on August 4th Levy had taken Until recently, that was where the out a patent application, part of which story ended. But all good tales have an was Laut's idea for heterodyne ampli- unexpected turn at the end, and this fication. Laut protested to Colonel one is no exception. Ferrie, whose response was that as they The 1991 Review of the American were at war, disagreements between Antique Wireless Association (AWA!) team members would be out of place. has a translation of a French article, Now it would be easy to assume originally published in 1979 in Liason that the accuracy of Paul Laut's rec- des Transmissions by Robert ollections had suffered from the pas- Champeix. In the article Champeix sage of 50 years; but there is unex- wrote that in 1968, he was involved in pected confirmation. setting up the French Radio and TV's At the end of 1925, and into the fol- museum, when an elderly but dapper lowing year, Lucien Levy carried on a and alert man introduced himself as heated debate with several radio per- Paul Laut. Laut had a very interesting sonalities via the correspondence story to tell. columns of the magazine L'Antenne. As an electrical engineer, Laut had (Compared with some of these been a member of the radio research exchanges, the arguments that Jim team set up in August 1914 under Rowe 'buys' in his Forum column are Colonel Ferrie at the Eiffel Tower, for models of politeness and affability). development of equipment using the The debate descended to personalities,