David Sarnoff, the head of RCA, had already decided to promote television vigorously and believed the company did not have the resources to develop a new radio medium at the same time. RCA engineers were impressed, but the sales and legal departments saw FM as a threat to RCA’s corporate position. With the four patents for his FM techniques that he obtained in 1933, Armstrong set about gaining the support of RCA for his new system. He soon found it necessary to use a much broader bandwidth than AM stations used (today an FM radio channel occupies 200 kHz, twenty times the bandwidth of an AM channel), but doing so gave not only relative freedom from static but also much higher sound-fidelity than AM radio offered. With frequency modulation, the audio signal alters instead the frequency of the carrier, shifting it down or up to mirror the changes in amplitude of the audio wave. In the usual technique, known as amplitude modulation (AM), the amplitude of the carrier wave is regulated by the amplitude of the audio signal. It is the only one I ever encountered that, approached from any direction, always seems to be a stone wall.” Armstrong eventually found a solution in frequency modulation, which is a different way of impressing an audio signal on a radio-frequency carrier wave.

In the early 1920s, Armstrong turned his attention to what seemed to him, and to many other radio engineers, as the greatest problem, namely, the elimination of static. It became - and remains today - the standard type of radio receiver. RCA marketed the superheterodyne beginning in 1924, and soon licensed the invention to other manufacturers. The essential idea is to convert the high-frequency received signal to one of intermediate frequency by heterodyning it with an oscillation generated in the receiver, then amplifying that intermediate-frequency signal before subjecting it to the detection and amplification usual in receivers. Armstrong used this principle in devising what came to be called the superheterodyne receiver. In 1901, Reginald Fessenden had introduced to radio the heterodyne principle: if two tones of frequencies A and B are combined, one may hear a tone with frequency A minus B. The first of these discoveries - of a powerful amplifier - vastly increased the sensitivity of radio receivers, while the second - of an oscillator - led to the use of the electron tube in transmitters and also in receivers for an added function, heterodyne reception.

He realized this meant that the circuit was generating its own oscillations, and thus that the triode could be used as a frequency generator.

Armstrong made a further discovery with this circuit: just when maximum amplification was obtained, the signal changed suddenly to a hissing or a whistling. Using a triode as an amplifier, he fed back part of the output to the input, and thereby obtained much greater amplification. Radio was Armstrong’s passion, and about a year before he graduated in 1913, he devised a circuit that revolutionized the radio art. Edwin Howard Armstrong (1890-1954) PHOTO: IEEE History CenterĮdwin Howard Armstrong was born in New York City on 18 December 1890, and studied electrical engineering at Columbia University under the celebrated Michael J. Our present FM radio owes more to Armstrong than to anyone else: he developed much of the technology himself, and he worked for two decades to establish FM broadcasting in the United States. Seventy-five years ago this month, on 26 December 1933, Edwin Howard Armstrong received four patents for frequency modulation (FM) inventions.

Edwin howard armstrong