The noise and signal that cause electronic equipment failure are both electrical energy. Electrical communication is the history of fighting against this difficult noise. However, today‘s information and communication technology has been established and our life has become colorful through the confrontation with the noise problem. In the future society where people are closely connected with high-quality services such as household appliances, automobiles and medical treatment, noise countermeasure technology will become increasingly important.
Aerial detection technology before the advent of radar
The radio waves used for communication are microwaves with wavelengths ranging from several centimeters to several millimeters. Although smart phones do not seem to have antennas, they are actually equipped with antennas for various purposes.
In the early 20th century, wireless communication used long wave to medium wave radio waves. Because of the low sensitivity of the receiver at that time, the antenna with a length of 100 meters to 1000 meters will be set up to a height of 100 meters above the ground. Such a huge antenna is also conspicuous in the distance. In wartime, wireless communication is the lifeblood of military. Therefore, in Europe during World War I, wireless communication antennas became the priority targets.
The discharge spark of the spark plug of the fighter engine will generate noise radio waves. Therefore, in World War I, in order to detect the approaching fighter, people invented superheterodyne technology to convert and detect the frequency of noise radio waves.
In the history of electrical communication technology in the 20th century, the American electrical engineer E. H. Armstrong served as an officer of the communication forces in World War I and went to the European battlefield. In order to protect important communication antennas, he came up with a clever plan to predict the coming enemy aircraft. As introduced earlier, Marconi‘s early wireless communication made use of the noise radio wave generated by the discharge spark, and the aircraft engine spark plug will also generate noise radio wave due to the discharge spark. His idea was that as long as he received the noisy radio waves, he could find the approaching enemy aircraft. However, the noise from the aircraft in the distance is a weak high-frequency wave. Therefore, he invented a method to convert the received high-frequency noise radio waves into low-frequency and then increase the amplitude for detection. This is the superheterodyne technology that has since been applied to radio, television and other devices.
Minimize the ground potential difference
The electronic equipment around us will emit noise more or less and will be interfered by external radiated noise. However, as long as the electronic equipment is wrapped in a metal shell, the noise in the form of electromagnetic waves will be blocked. This is because the metal shell will not receive noise waves and put them into it. 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 noise radio wave energy and further improve the effect. The electromagnetic shielding room used for noise measurement of electronic equipment is also coated with metal and connected to the ground to block external noise.
Although we often say "Earth", accurately speaking, the metal shell and chassis of electronic equipment and the circuit board used for signal return circuit are actually "Ground". In addition, the metal shell and chassis are called Frame Ground, and the circuit board is called Signal Ground.
In terms of electronic equipment, it is necessary to strictly distinguish Earth from Ground. This is because even if Ground is grounded, there is still a small difference between the reference potential of Ground and Earth, which will cause noise. This also happens between Frame Ground and Signal Ground connected with analog circuit, digital circuit and power circuit. It is necessary to reduce the potential difference between the grounds by setting reference points and other detailed operations.
The circuit board sometimes becomes an antenna radiating noise
"In the right place, Ground needs to be as thick and short as possible." This is the first principle of circuit board design. The reason why the Signal Ground is connected to the chassis and other frame grounds is to increase the area of the ground. However, if the connection is made by means of screws or the like, and the screws are loose, there will be radiation noise, so pay more attention. The daisy and spring gaskets with serrations are also used to ensure better contact. In addition, in terms of circuit boards, it is also important to note that "signal circuits and return circuits as Signal Ground cannot form a large area circuit cycle". This is because this cycle will form an antenna and generate high intensity radiation noise. There are many printed circuit boards for electronic equipment. The reverse and inner layers are Signal Ground, and the signal circuit is on the front. The return circuit flowing in Signal Ground will select the shortest distance with the lowest resistance. Therefore, it will flow directly below the signal circuit, which can reduce the area of antenna loop and suppress the generation of radiated noise.
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