A Selection of Receiver Innovation History over the Past 100 Years (Part 1)

Figure 1. Marconi demonstrating his technique.

Although the technology he commercialized was not the latest and was evolving rapidly, it was good enough because he had a way of figuring out how to use existing technology to create a new industry. The early 20th century saw the end of colonialism, wars and disasters; in April 1912, the RMS Titanic sank. Amid this world chaos, Marconi set out to deploy a global network to send and forward information wirelessly. After the Titanic sank, wireless technology played a positive role in rescuing survivors and spreading news of the accident, raising the importance of this emerging technology.

Radios in Marconi's day were, as far as we can guess, very primitive. The transmitter used a spark gap device (mechanical AC generators were used later) to generate the RF, but on the receiving end, the system was completely passive, consisting of an antenna, a resonant LC tuner, and some kind of detector. We'll discuss these detectors shortly, but at the time, they could be mechanical, or chemical or organic. Some of these systems simply biased them with batteries, but did not provide any circuit gain, unlike today. The output of these systems was fed into some kind of headset, which converted the signal into audio, which was always very weak, no more than a simple click or buzz.

Because these systems offered no gain on the receiving end, their effective range depended on the amount of transmitted power, the quality of the receiver, the operator's experience in tuning, and of course, atmospheric conditions. Marconi realized that with reasonably predictable effective range, a network of stations could be built to reliably relay information across continents and oceans. This involved installing equipment both on land and at sea. Marconi began installing radio stations around the world and at sea, including on passenger and cargo ships. By installing radio systems on seafaring vessels, he not only enabled those ships to communicate with their commercial stakeholders on shore, but also provided relays and redundancy where necessary, allowing Marconi to fill critical gaps in his network.

One technology Marconi had was an early vacuum tube. John Ambrose Fleming, the recognized inventor of the vacuum tube, worked for Marconi, but Fleming and Marconi analyzed that their existing technology was sufficient to detect radio signals. In addition , they felt that his discovery, while beneficial, was not worth the extra money or batteries required to run the valve tubes. Marconi already had several signal detection technologies that, unlike the valve tubes, did not require high power to run the filaments and heating plates. So they initially abandoned that technology.

However, Lee de Forest, the so-called father of radio, picked up on the technology and realized its potential. By inserting a screen grid between the filament and the heating plate, he could not only rectify the signal, but also control the amount of current in the heating plate. This achieved amplification. Although there is evidence that he did not understand how his triode worked, he did realize its potential and tried to take advantage of the invention, not only as a technology, but also as a value-added service similar to Marconi's invention.

By establishing various businesses, de Forest tried to manufacture and sell his vacuum tubes and to build a wireless network similar to Marconi's. However, these businesses were doomed to failure, not because the technology was bad, but because de Forest's business partners were often less than honest and often left him alone to take the blame for other people's mistakes. In the end, de Forest had to sell the rights to his invention so that others could enjoy the profits.

Edwin Armstrong was one of the early pioneers in recognizing the possibilities of vacuum tubes. While still in high school, a family friend gave him a De Forest triode to play with. Armstrong, who had already gained a reputation as an expert in wireless technology and had built his own radio at home, quickly figured out how to use the device to develop a better receiver. While in college, he continued to develop the technology and developed a regenerative receiver that offered superior performance compared to the passive systems used by all radios at the time. David Sarnoff was a senior figure at the Marconi Corporation in the United States. His long-established relationship with Marconi himself and his dedicated work ethic enabled him to rise quickly in the company. Sarnoff started out running errands for AMC and met Marconi by chance during one of his visits to the United States. Marconi impressed Sarnoff, who set the stage for his advancement at the company, and eventually Sarnoff became a senior leader at AMC and then RCA. He met Armstrong by chance while visiting the New York Engineering Laboratory. The two developed a long-lasting professional and personal relationship, thanks to Armstrong's extensive knowledge of wireless technology and the power of his regenerative receivers.

When World War I broke out, Armstrong felt the call of duty and enlisted. But by then, he had already developed a reputation as an expert in wireless technology, so instead of being sent to combat duty, he was sent to France to overhaul and install radio stations for combat troops throughout the country. His duties gave him access to equipment, laboratories, and a variety of techniques, and allowed him to continue his research activities on the side. During an air raid in early 1918, he made a series of discoveries that led him to synthesize a superheterodyne receiver. Throughout 1918, he worked hard to develop his concept, and by November he met with a group of close friends to demonstrate a prototype of a superheterodyne radio. His friends were impressed and urged him to continue development. By the end of 1918, the war was about to end, and before returning to the United States, Armstrong applied for a French patent on December 30, 1918. After returning to the United States, he took several weeks to recover from an illness that delayed his filing of a U.S. patent application. Finally, on February 8, 1919, he applied for a U.S. patent for the superheterodyne receiver.

While Marconi's vision for wireless technology focused on business messages carried by telegraphs between two parties, Sarnoff's vision was much broader—sending signals to many parties. Sarnoff's vision was not widely shared at first, but others eventually realized that the new technology offered a way to easily transmit news and entertainment over long distances, including to rural areas of the United States. To further this vision, Sarnoff and his team came up with a plan to broadcast the Dempsey-Carpentier boxing match on July 2, 1921. The success of the broadcast event led others to see the potential for broadcast radio as we know it today. However, the real challenge at the time was technical.

Early radios were difficult to use and did not function well. The story of Armstrong, Sarnoff, and RCA continues from here. Through previously developed relationships and RCA patents, including the superheterodyne receiver patent, radio technology has been greatly simplified to become portable and accessible to everyone. From a technical perspective, the superheterodyne architecture was the key to this achievement, and it remains largely the same today.

Somehow a radio must produce an output signal that carries meaningful information. In the early days, this was the resonant spark produced in the receiving loop antenna. It was soon realized that a more sensitive way of converting radiated energy into a meaningful signal was needed. Early technologies were very limited and typically exploited multiple properties, including chemical, mechanical, and electrical.

One of the first detectors used was what was called a metal chip detector, based on the discoveries of a Frenchman called Édouard Branly. The metal chip detector consisted of two metal plates, a small space between them, into which a certain amount of metal powder was injected. When an RF signal reached the plates, the powder would stick to them, closing the circuit. This worked very well for detection, but once the RF signal was removed, the powder would continue to stick to the plates. To solve this problem, some kind of knocker was arranged to knock on the side of the device, forcing the powder off the plates. For this reason, this primitive detector, while effective, was very cumbersome to use. Despite this, people were still using it as late as 1907.

Figure 5. Metal chip detector

Figure 6. Schematic diagram of metal chip receiver

A more practical solution was the electrolytic detector. This device consisted of a very thin platinum wire immersed in a solution of sulfuric or nitric acid. A battery was used to bias the circuit to the point of electrolysis. This formed bubbles on the surface of the platinum wire, causing the current to drop. If an RF current was coupled into the circuit, it would modulate the electrolytic point and cause the current to vary with the strength of the coupled RF signal. This technology was developed by Fessenden and was widely used between 1903 and 1913. De Forest developed a variation on this technology, called a transponder, consisting of two metal plates immersed in a lead peroxide solution.

Marconi preferred another scheme known as a magnetic detector. These devices were affectionately known as maggies by their users. They worked by forming an endless loop of steel wire and magnetizing it with the aid of a permanent magnet while rotating it in a circle. The magnetized portion of the wire passed through a coil connected to an antenna. The RF field in this coil demagnetized the wire depending on the level of received signal present. The changes in the wire's magnetic field were then picked up by another coil, which was connected to headphones that provided an audible RF signal. All Marconi devices until 1912 used this scheme, including the one on the Titanic.

Another common type of detector is the crystal detector, which was popular until 1925. This popular device is often called a cat whisker, which is basically an early semiconductor junction made from various minerals. Typical minerals include galena (PbS), pyrite (FeS2), molybdenite (MoS2) and silicon carbide (SiC).

Small samples of these rocks were made in metal cups and a fine wire was used to make a point contact on the rock. This point contact could be moved around and placed in various locations on the rock to find the best working point. There are still crystal radios on the market today that use exactly the same circuits as they did 100 years ago, except that a manufactured semiconductor diode has replaced the whisker. One advantage of crystal detectors is that these units offer more linear detection, which became very important at the beginning of AM broadcasting. This made voice communication possible, whereas early transmissions were sent only in Morse code.

Another type of detector was built by an engineer working for Marconi in 1904. John Ambrose Fleming discovered that by adding a plate to an Edison incandescent light bulb, he created a device commonly known as a rectifier or valve. Marconi and Ambrose decided that their existing detector scheme (usually the Magi) performed better than Fleming's valve, so they temporarily suspended their efforts to find a better solution until after 1912.