9/18/2007 Oscillators notes 1/2 Jim Stiles The Univ. of Kansas Dept. of EECS D. Oscillators The invention of the electronic oscillator was one of the most important steps in creating modern radio. Q: Guess who invented it? A: Edwin Howard Armstrong! HO: Oscillators-A Brief History HO: Oscillators Oscillators, like all other devices we have studied have many non-ideal properties. HO: Harmonics, Spurs, and dBc Another non-ideal property of oscillators is instability. To understand fully this property, we need to re-discuss what you think you know about phase and frequency! HO: Phase and Frequency HO: Oscillator Stabilty Perhaps the most important, but least understood characteristic of any oscillator is an instability known as phase noise.9/18/2007 Oscillators notes 2/2 Jim Stiles The Univ. of Kansas Dept. of EECS HO: Phase Noise For a such a simple device, an oscillator has many potential problems. Among some others are frequency pushing and frequency pulling. HO: Pushing and Pulling Let’s summarize what we’ve learned: HO: The Oscillator Spec Sheet9/18/2007 Oscillators A Brief History 1/4 Jim Stiles The Univ. of Kansas Dept. of EECS Oscillators-A Brief History In September of 1912, Edwin Howard Armstrong was experimenting with Lee DeForest’s new device—the audion (what we now call the triode vacuum tube). These devices had been successfully used as an AM detector, but no one (especially DeForest!) quite knew how or why the device worked, or what other applications of the device there might be. By coupling one terminal of the device to another, Armstrong found that he could achieve large signal gain—he had built the first electronic amplifier! He called the process “regeneration”; we know it today as positive feedback. The electronic amplifier would revolutionize radio, but Armstrong was not yet finished! The Triode Vacuum Tube9/18/2007 Oscillators A Brief History 2/4 Jim Stiles The Univ. of Kansas Dept. of EECS Armstrong found that as he adjusted his amplifier to achieve maximum gain, the circuit would suddenly begin to “squeal”. Of course, this was disappointing at first, but then Armstrong realized this “squeal” was a high-frequency signal—the circuit was oscillating! Armstrong had of course increased his feedback to the point that the circuit had become unstable—his poles where in the right-half plane! Armstrong had made the first electronic oscillator—this too would revolutionize radio! Armstrong had created the components necessary to make Continuous Wave (CW) radio practicable. Recall that radio at that time was primarily wireless telegraphy (i.e., dots and dashes). CW radio is required to transmit audio information (e.g., music and voice). Engineers had already created some CW radio systems, using electromechanical oscillators, but they could create signals only in the kHz range at best. With Armstrong’s oscillator, CW signals at high frequencies (e.g. kHz and MHz) could be easily generated! Doh!9/18/2007 Oscillators A Brief History 3/4 Jim Stiles The Univ. of Kansas Dept. of EECS Along with the amplifier, the electronic oscillator allowed for the creation of reliable, “low-cost” radio systems with clear and audible sound! Although these inventions gave a tremendous boost to the radio industry, a major technical problem still remained. But guess what? Armstrong would solve this problem too! ARMSTRONG, EDWIN HOWARD (Dec. 18, 1890 -- Jan. 31, 1954), electrical engineer and inventor of three of the basic electronic circuits underlying all modern radio, radar, and television, was born in New York City, the first child of John and Emily Smith Armstrong, both native New Yorkers. His mother had been a teacher in the public schools and his father was vice president of the United States branch of the Oxford University Press. The family soon moved to the suburban town of Yonkers, N.Y., where they lived in a house on a bluff overlooking the Hudson River. Armstrong decided to become an inventor when he was fourteen and began filling his bedroom with a clutter of homemade wireless gear. His imagination was fired by the Boy's Book of Inventions and by Guglielmo Marconi, who a few years before had sent the first wireless signals across the Atlantic. But wireless telegraphy was still in a primitive state. Its crude spark-gap transmitters produced electromagnetic wave signals so weak that sunlight washed them out through most daytime hours, while its iron-filing or magnetic receivers were cruder still, requiring tight earphones and quiet rooms to catch the faint Morse code signals that were all the early wireless was capable of transmitting. As a student at Yonkers High School (1905-1910), Armstrong built an antenna mast, 125 feet tall, on the family lawn to study wireless in all its aspects. He worked with every new device that came along, among them the so-called audion tube invented in 1906 by Lee deForest. But none of the instruments were able to amplify weak signals at the receiver, nor yet to provide stronger, more reliable power at the transmitter. On graduating from high school, Armstrong began to commute by motorcycle to Columbia University's school of engineering to pursue his studies further. While a junior at Columbia, Armstrong made his first major invention. Long analysis of the action within the audion tube suggested to him that it might be used to greater effect. The tube was based upon Thomas Edison's 1883 discovery in his early lamp of a tiny anomalous electric current that flowed across a gap from the filament to a metal plate. In 1904 an English inventor, John Ambrose Fleming, had shown that this effect could be used as a wireless receiver, two years later deForest had added a vital element, a wire grid between the filament and plate. But in the usual receiver circuit the tube did no more than detect weak signals. In the summer of 1912 Armstrong devised a new regenerative circuit in which part of the current at the plate was fed back to the grid to strengthen incoming signals. Testing this concept in his turret room in Yonkers, he began getting distant stations so loudly that they could be heard without earphones. He later found that when feedback was pushed to a high level the tube produced rapid oscillations acting as a transmitter and putting out electromagnetic waves. Thus this single circuit yielded not only the first radio amplifier but also the key to the