A large number of EMI radiation sources can cause serious damage to the circuit

Electromagnetic interference (EMI) is a part of our life. With the passage of time, a large number of radiation sources of intentional and unintentional EMI will cause serious damage to the circuit. The signals of these radiation sources may not pollute the circuit, but our purpose is to keep the low-noise system away from these hazards.

We can imagine that doctors use ECG diagnostic equipment to accurately diagnose the heart. After knowing that this is a high-precision measuring device, we will not worry about the annoying noise in the diagnosis results. This is a low frequency measurement, and the electronic equipment does not exceed 1MHz. However, if an ECG device with poor EMI design is used, and the doctor answers the phone with his mobile phone during the examination, then there is reason to worry about the diagnosis result.

Figure 1. Cardiac examination results of ECG diagnostic equipment when the transmitter beyond 1.5 feet (F=470 MHz, P=0.5W) is turned on and off

In Figure 1, the cardiac input signal of the system is about 0.25mVp-p. This small signal needs about 60000 V/V to measure the amplifier gain. Fortunately, the situation shown in Figure 1 does not mean the actual performance of ECG measuring equipment for medical purposes. This measurement is actually using the circuit board shown in Figure 2 in the engineer's laboratory.

Figure 2. Front view of precision low level ESG heart rate meter circuit board

Don't fall into this EMI trap. Carefully build the circuit board and use some resistance EMI which is independent of the bandwidth of analog or digital circuits. When there is an application near the application circuit, the EMI radiation source may or may not affect it.

When using this low-frequency circuit board, how does the radiated noise of the mobile phone enter the measurement results (see Figure 1)? Let's review and study the EMI diagram of the whole process. There are three factors in EMI: radiation source, coupling pathway of radiation signal transmission and radiation receptor. In this case, the radiation source is obvious. EMI signal source can be transmitted wirelessly through air or through PCB, and the radiation source is unknown.

EMI (also known as radio frequency interference, RFI) receptors are surrounded by direct conduction or various field transmission. These fields are directly coupled to the circuit connection line and PCB in the circuit, and converted into conductive RFI.

The formation of a force between two charges requires three conditions, magnetism and electromagnetic field (radiation). The electric field (volts/distance) describes the force formed by the uneven charge distribution between two physical points. In order to balance the charge distribution, a force is formed between the charges.

The moving charge or current forms a magnetic field and exerts a force on all other charges around it. This field (or force) decreases rapidly as the distance increases. Please note that the electric field and magnetic field are interrelated, one change, the other change.

Finally, the acceleration of electrons (or charges) forms an electromagnetic field. This kind of electromagnetic field is the most common cause of EMI transmission.

Is there a way to solve this problem? Next time, we will discuss the characteristics of some radiation sources and introduce some tips to teach you how to minimize this radiation. Please look forward to it.