EMC Testing - Configuration and basic steps of radiated emission tests

Why do we make electronic equipment? EMC EMC test and how EMC does the test are hardware from beginning to end. PCB electronic design is an indispensable part of engineers. According to statistics, radio and telecommunication products in the vast majority of countries in the world should undergo EMC test before becoming qualified electronic products and circulating in the market. The purpose of the EMC test process is to check whether the interference signal transmitted by the goods is within the acceptable range and ensure that your products and functions will not be adversely affected by the interference signal.

Electromagnetic compatibility (EMC) is so important. What do we need to do in the project test of EMC products?

EMC testing of a complete product project includes the following processes:

EMC radiation emission test: interference signal measurement (EMI) is tested according to electrical equipment

EMC immunity test: measure the sensitivity of goods to electromagnetic interference from other sources

Electrostatic discharge (ESD) test: discharge static electricity when two objects contact. Examples of ESD include sometimes feeling static electricity after drying clothes in a clothes dryer or in lightning.

Conducted immunity test: introduce current into the input and output cables of the system under test

Conducted emission test: RF energy transmitted from electrical equipment to electrical system according to power line

Frequency emission compatibility test: frequency emission is the effective number of electromagnetic radiation energy for communication. The test provided by us focuses on the normal operation of the antenna connected to the RF receiver/transmitter subsystem in the system, which will not cause the level from antenna to antenna to drop

Today we will explain the EMC radiation emission test in detail. Conducted emission test

EMC Test - Radiated Emissions

Definition: The radiation interference intensity of key radiation emission test equipment to the outside during normal operation shall be arranged according to the typical installation requirements of the client, and the cable shall comply with the length and model specified in the specification; Except for the equipment specified in 36.201.1a, other equipment and systems shall be classified and tested based on GB4824 (Industrial, Scientific and Medical (ISM) Classification Equipment Disturbance Characteristic Limits and Measurement Methods).

Category: the frequency of radiation emission is 9kHz - 18GHz

(1) 9KHz-30MHz frequency band

Measure the magnetic field H in this frequency band. When the EUT is small, place it on the large magnetic ring antenna (LLA) to measure the induced current disturbance magnetic field; When the EUT is large, select the far antenna method and measure the magnetic field intensity of the disturbance at the specified distance with a single small ring.

(2) 30MHz - 18GHz frequency band

In this frequency band, the measuring electric field E is below 1GHz, that is, 30MHz - 1GHz. The places where equipment is required are roughly as follows: select a wide place or a semi-anechoic chamber, simulate a semi-free space, a 00.8m high wooden table, a 360-degree rotary table, a measuring distance of 360 degrees, 10m antenna 1-4m high adjustable, antenna vertical polarization and horizontal polarization test, broadband antenna, coaxial line. 50 disturbance detectors Ω paired transmission system, Antenna impedance, coaxial impedance and input impedance affecting the detector shall be the same. Impedance mismatch will cause reflection, generate standing wave, and endanger the accuracy of reading. Distinguish the characteristics of disturbance detector and select quasi-peak or peak measurement.

Above 1GHz, that is, 1GHz - 18GHz, the place where equipment is required is roughly as follows: in the full anechoic chamber, simulate the free space, set the receiving antenna in the EUT highly unified, the turntable still needs 360 degrees of rotation, and the test spacing is 360 degrees m. Select a small aperture fixed antenna, measure the horizontal and vertical conditions, and the minimum distance of the antenna should be more than 25cm. In order to avoid causing antenna performance, select a spectrum analyzer with large maintenance mode and most dB dynamic display, For the strong peak or average value identification of the measurement results (no quasi-peak value is required), select 1MHz for peak measurement. In the average measurement, the resolution bandwidth and video bandwidth still select 1MHz resolution bandwidth, but the video bandwidth should be greatly reduced to 10Hz, which is equivalent to adding a low-pass filter.

Radiated emissions - test process:

Handle the antenna and receiver at 3m from the EUT (test equipment) wall, and adjust the antenna core to a height of 2m above the ground; First, shut down the test equipment and test the ambient noise level; Turn on the test equipment, adjust the wiring mode (including the antenna height), the polarization direction of the antenna and the angle of view of the turntable), and select a larger radiation point; Test the different functions of the equipment and find a larger radiation mode; Rotate the antenna to test horizontal and vertical radiation disturbance respectively; With the test equipment as the core mobile antenna, select as many test points as possible for measurement; Select the maximum radiation level as the measurement result; The experiment can be carried out in a wide field and a semi-anechoic chamber; Turn off the device.

EMC Test - Conducted Emissions

Conducted emission test, commonly known as CE, also known as conducted disturbance, refers to the electromagnetic phenomenon that the system transmits to other software or equipment interference sources according to the internal voltage or current of signal cable, power line or ground wire. Conduction and emission tests are commonly referred to as disturbance voltage tests. As long as there is a power cord, it will involve products, including many DC power products. In addition, in many specifications, the signal/control line also needs to transmit and transmit, which is usually expressed by the limit of the disturbance voltage or the disturbance current (there is correlation between them).

Conducted emissions - Test equipment - Process:

The EUT shall be placed on a strong test bench with a distance of 80 degrees to 90 cm from the road surface. The surface of the test bench is a conductive or non-conductive plane, and a metal ground plate is laid. Generally, the environment in which the EUT is actually used. Select a conductive or non-conductive test bench. For example, the portable equipment can be placed on the non-conductive test bench, and the equipment installed in the ship's warehouse should be tested on the metal conductive test bench. Receive the power line to be tested according to the stable network of power impedance. The cables to be tested can be placed according to standards and laid in different lengths.

The experimental configuration of conducted emission test standard for table-type equipment is as follows.

① Extra long cable, long cable, folded into 30~40cm harness;

③ The projection distance between EUT and LISN ground is 80cm;

④ The spacing between each cable and GND is more than 10cm;

⑤ The distance between the end of the suspended cable and GND is more than 40cm to ensure that there is not too much coupling space.

The experimental configuration of the vertical equipment experimental standard is as follows.

③ EUT and GND shall be insulated;

④ The spacing between each cable and GND is more than 10cm (this has a great impact because the common-mode current in the cable will vary greatly).

The purpose of the conduction and emission test is to evaluate and regulate the level of conducted electromagnetic interference emitted from the control line or ground wire according to the unified standard (limit value) and electronic equipment. power line.

When the noise frequency of electronic equipment is lower than 30MHz, the audio frequency band will be affected. For this low-frequency electromagnetic wave, the cable of electronic equipment is not at the wavelength of (30) MHz (10m), and its radiation efficiency to the air is very low. In this way, if the induced noise voltage on the cable can be measured, the electromagnetic noise impact level of this frequency band can be considered. This kind of noise is conductive noise, just like interruption noise. The click of iron, floor washer and rice cooker. The influence noise measured here is only for the continuous disturbance voltage within the 30 MHz frequency test. As shown in Figure 1.1 below, the experimental equipment is used for the conducted disturbance test of the input port, LISN linear impedance stable network, EUT is the test equipment, and LOAD is used for the load of the output port. Figure 1.2 below shows the experimental equipment for conducting the conducted disturbance test at the output port.