The noise that causes electronic equipment to malfunction, like the signal, is electrical energy. Electrical communication is the history of the struggle against this intractable noise. However, by confronting the problem of noise head-on, today’s information and communication technology has been established, and our lives have been enriched by it. In a future society where people and high-quality services such as home appliances, automobiles, and medical care are closely linked, noise countermeasure technology will become increasingly important.
The radio waves used for communication are microwaves with wavelengths ranging from several centimeters to several millimeters. Although smartphones appear to have no antennas, they actually have built-in antennas for various purposes.
Wireless communications in the early 20th century used long-wave to medium-wave radio waves. Due to the low sensitivity of the receiver at that time, the antenna with a length of 100 meters to 1,000 meters was erected to a height of more than 100 meters above the ground. Such a huge antenna is also conspicuous from a distance. In wartime, wireless communications are the lifeblood of the military. Therefore, in Europe during the First World War, wireless communication antennas became the object of priority attack.
The discharge spark of the spark plug of a fighter jet engine will generate noise radio waves. Therefore, in World War I, in order to detect approaching fighter jets, people invented the superheterodyne technology to convert and detect the frequency of the noise radio waves.
E.H. Armstrong, an American electrical engineer who left his name in the history of electrical communication technology in the 20th century, served as an officer of the communications force during World War I and went to the European battlefield. In order to protect important communication antennas, he came up with a clever way to predict incoming enemy planes. As mentioned earlier, Marconi’s early wireless communication used the noise waves generated by the discharge sparks, and the aircraft engine spark plugs also generated noise waves due to the discharge sparks. His idea is that as long as the noise waves are received, the approaching enemy aircraft can be detected. However, the noise from distant aircraft is a faint high-frequency wave. Therefore, he invented a method of converting the received high-frequency noise waves into low-frequency waves, amplifying them, and then detecting them. This is the superheterodyne technique that has since been applied to devices such as radios and televisions.
Electronic devices around us are more or less noisy and are disturbed by external radiated noise. However, as long as the electronic device is wrapped in a metal case, the noise that flies in in the form of electromagnetic waves will be blocked. This is because the metal case does not pick up the noise wave and put it inside. Among the four noise countermeasures of shielding, reflection, bypass, and absorption, this shielding method is best understood. At this time, if the metal shell is connected to the earth, the earth can absorb the energy of the noise radio wave and further improve the effect. The electromagnetic shielding room used for noise measurement of electronic equipment, etc. is also covered with metal to block external noise and connected to the ground.
Although we often say “earth”, to be precise, the metal shell and chassis of electronic equipment, the circuit board used for signal return lines, etc. actually belong to “ground”. In addition, the metal shell and chassis, etc. are called Frame Ground, and the circuit board is called Signal Ground.
In terms of electronic equipment, it is necessary to strictly distinguish between Earth and Ground. This is because even if Ground is grounded, there is a slight difference between the reference potentials between Ground and Earth, which causes noise. This situation also occurs between the Frame Ground and the Signal Ground connected to analog circuits, digital circuits, and power circuits. It is necessary to reduce the potential difference between the grounds by careful operations such as setting reference points.
“In the right place, the Ground needs to be as stubby as possible.” This is the first principle of board design. The reason why the Signal Ground is connected to the Frame Ground such as the chassis is to increase the area of the Ground. However, if it is connected by screws, etc., and the screws are loose, radiation noise will be generated there, and more attention should be paid to it. Serrated chrysanthemum washers and spring washers are also used to ensure better contact. In addition, on the circuit board side, it is also necessary to pay attention to “not allowing the signal circuit and the return line as the Signal Ground to form a large-area circuit loop”. This is because such loops can form antennas that generate high levels of radiated noise. Electronic devices have many printed circuit boards with Signal Ground on the back and inner layers, and signal circuits on the front. The return circuit flowing in Signal Ground selects the shortest distance with the lowest resistance. Therefore, it flows directly under the signal circuit, which reduces the area of the antenna loop and suppresses the generation of radiated noise.