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Application of TEM Chamber in EMC Test
Release time:
2022-07-28 00:00
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1. Overview of TEM cell
Introduction
The TEM cell, also known as the transverse electromagnetic wave chamber, is a special rectangular cross-section transmission line composed of a horn-shaped outer conductor at both ends that gradually shrinks and an inner conductor with a strip-shaped partition in the middle. In the frequency range from DC to 200MHz, the transverse electromagnetic wave chamber is mainly used for radio frequency electromagnetic field radiation immunity test and radio frequency electromagnetic field emission measurement of electronic equipment. In order to ensure the transverse electromagnetic field generated in the transverse electromagnetic wave chamber, the size of the transverse electromagnetic wave chamber is limited by the upper working frequency. Therefore, the transverse electromagnetic wave chamber is more suitable for the radio frequency electromagnetic field immunity test and radio frequency electromagnetic field emission of the equipment under test (EUT) for circuit modules, printed circuit boards and small electronic devices.
basic structure
The middle part of the outer conductor of the transverse electromagnetic chamber is spliced by a top plate, a bottom plate and two side plates. The two ends gradually shrink in the shape of a horizontal rectangular cross-section horn, and are connected with the outer conductor of the coaxial cable with a characteristic impedance of 50Ω to form a metal enclosure to isolate the electromagnetic environment inside and outside the transverse electromagnetic wave room and ensure that the external electromagnetic environment does not affect the indoor environment. Measurement, the indoor electromagnetic field is confined indoors without affecting the outdoors.
The middle part of the inner conductor of the transverse electromagnetic wave chamber is a strip-shaped partition, and the two ends of the partition shrink gradually and connect with the inner conductor of the coaxial cable with a characteristic impedance of 50Ω. The inner and outer conductors of the transverse electromagnetic wave chamber make the electromagnetic energy transmit from one end of the transverse electromagnetic wave chamber to the other end in the transverse electromagnetic field mode. In order to avoid standing waves in the transverse electromagnetic wave chamber, the two ends of the transverse electromagnetic wave chamber are required to be matched with coaxial wires with a characteristic impedance of 50Ω.
Usually, one end of the transverse electromagnetic wave chamber is connected to a 50Ω load impedance, and the other end is connected to the electromagnetic field excitation source or receiving device through a coaxial cable with a characteristic impedance of 50Ω, which mainly depends on whether to do radio frequency electromagnetic field immunity test or radio frequency electric field emission measurement . During the test, the device under test is placed between the top plate and the partition or between the partition and the bottom plate, and the device under test is supported by an insulating material with a relative permittivity close to 1.
In order to facilitate the placement of the equipment under test, the transverse electromagnetic wave chamber is generally designed with a small electromagnetic shielding door that has good electrical contact with the surrounding area. Compared with anechoic chambers, electromagnetic reverberation chambers and Guhertz transverse electromagnetic wave chambers (GTEM chambers), transverse electromagnetic wave chambers are relatively cheap and easy to move. Since the transverse electromagnetic wave chamber does not use an antenna, the voltage of the inner and outer conductors can vary from a few microvolts to hundreds of volts, so the transverse electromagnetic wave chamber can also be used to conduct electromagnetic pulse (EMP) tests on the equipment under test.
Accessory equipment
Including an electromagnetic wave signal generator capable of covering the specified frequency range and frequency sweep function and capable of being 80% amplitude modulated by a 1kHz sine wave, a power amplifier that amplifies the signal (unmodulated and modulated) to a specified field strength level, a power meter, and a voltmeter , a two-way combiner for incident and reflected signal measurements. An attenuator, an analysis and recording instrument corresponding to the power level of the field strength and a monitoring system of the equipment under test [1].
2. Radiation emission TEM chamber method of components/modules
1 Overview
Measurements of radiated field strength should be performed in shielded enclosures to eliminate high levels of extraneous interference from electrical equipment and radiated fields in the vicinity of broadcast and other radio transmitters. The TEM cell serves as a shielded enclosure. Figure 1 shows an example of a TEM (Transverse Electromagnetic Mode) cell.
1 outer shield
2 separator (inner conductor)
3 access doors
4 connector panel (optional)
5 coaxial connectors
6 EUTs
7 Low relative permittivity bracket (ε≤1.4)
8 artificial harness
If the RF boundary extends outside the TEM cell, the connectors on the connector panel shall be coaxial RF connectors.
Figure 1 - TEM cell (example)
The upper frequency limit of this test method is a direct function of the TEM cell size. Dimensions include the dimensions of the component/module as well as the layout and characteristics of the RF filter. Measurements must not be made near the resonant frequency of the TEM cell.
The TEM cell is recommended to be used in automotive electronic systems in the test frequency range of 150 KHZ to 200 MHZ. In order to obtain reproducible test results, the EUT and test harness should be placed in the TEM chamber in the same position for each repeated measurement. In this experiment, the diaphragm of the TEM cell functions like a receiving antenna.
2 Test layout
2.1 Main field emission arrangement from wiring harness
The TEM cell should have a connector panel as close as possible to the multi-connector (see Figures 2 and 3)
1 TEU
2 Low relative permittivity support (ε≤1.4)
3 PCB or wiring harness
4 connectors
5 coaxial connectors
6 connector panel (optional)
7 TEM chamber wall
8 RF coaxial cable
All wires leading to the EUT shall pass through the RF boundary. The RF boundary is either on the TEM chamber wall or extends through the RF coaxial cable (8) and coaxial connector (5). The boundary is terminated by a radio frequency filter which can be connected inside the connector panel (6) or directly outside the coaxial connector (5). If the RF filter is connected to the coaxial connector (5), the cable in the connector panel should be coaxial.
Figure 2 - Example of wire arrangement in TEM cell connector panel
1 TEU
2 Low relative permittivity support (ε≤1.4)
3 PCB (no ground plane) or wiring harness, no shield
4 connectors
5 coaxial connectors
6 connector panel (optional)
7 TEM cell wall
8 cables
9 partitions
b is the height of the TEM cell
If the RF boundary extends outside the TEM cell, the connectors on the connector panel shall be coaxial RF connectors.
Figure 3 - Example of arrangement of connector, lead frame and dielectric holder
All power and signal leads of the EUT are directly connected to the artificial wiring harness (e.g. lead frame). Unneeded plugs on the connector panel should be sealed so that they are RF tight. The connection of the positive power lead should be through the artificial mains network, directly to the connector panel. The EUT is not allowed to be directly grounded to the TEM chamber ground. Grounding should be done at the connector panel.
2.2 Main field emission arrangement of EUT
The test setup is similar to the method described above, except that the leads of the EUT are positioned and shielded to minimize electromagnetic emissions from the leads. Do this by laying the lead flat on the bottom of the TEM chamber and placing it vertically on the device under test. The use of sealed cells and shielded wiring in the TEM cell will further reduce electromagnetic radiation from power and signal leads. To further reduce radiation from the lines, shielding foil tape can be used on the wires.
2.3 Power and artificial power networks
Artificial mains network with coaxial connector will facilitate connection to TEM cell EUT power connector
2.4 Signal/Control Line Filters
In the TEM cell test method using EUT lead coaxial connectors, each lead should pass through a filter whose impedance characteristics are similar to those of the lower artificial mains network.
The attenuation of the filter should be specified for the entire frequency range over which the component/module is expected to be tested, according to the requirements shown in Figure 4. The attenuation shall be: more than 40 dB from 30 MHz to the upper cut-off frequency fc, depending on the intended test method. Figure 4 shows the test method for the selected upper cut-off frequency fc = 400 MHz.
Note 1: Other low-pass RF filter configurations may be used if the filter characteristics are not suitable for specially required signals at the input or output of the EUT (eg high-speed network data interface). Filters can be specified in the test plan.
Fig.4 - Example of minimum attenuation required for signal control line filter
The attenuation of such a filter can be determined by a two-port network analyzer measurement (S21). The input and output impedance of the network analyzer should be 50 ohms.
The test setup is shown in Figure 5.
Note 2: An equivalent method of measuring in a 50 ohm system, such as a measuring receiver or an equivalent device with a built-in tracking generator, can also be used to measure only the magnitude of the attenuation.
Figure 5 - Filter attenuation measurement setup
4 Test procedure
Figure 6 shows an example of the test layout for the TEM cell method. General scope of EUT, wiring harness, TEM wall filter system, etc. Indicates standard test conditions. Any deviation from the standard test configuration shall be confirmed prior to testing and documented in the test report.
1 measuring instrument
2 TEM cells
3 TUEs
4 Artificial power network
5 power supply
6 50 ohm termination resistor
7 Low relative permittivity support (ε≤1.4)
Figure 6 : Example of TEM chamber test setup
The device under test shall be supported b/6 above the TEM chamber floor by non-conductive, low relative permittivity material (ε≤1.4) within the allowable working area (see Figure 3). The length of artificial wiring harness (such as lead frame) should be (450±45)mm, and shown in Figure 2 and Figure 3. The wiring and design of the artificial wiring harness and the overall height of the DUT connector constitute electrical coupling loops and dipoles that affect the test results. All connections between the EUT (multipole) connectors and the plugs and contacts of the artificial harness shall be as short as possible. Repeat measurements shall be made with the same artificial harness arrangement, the same overall EUT connector height and the same pin assignments on both connectors. Care should be taken if the dimensions of the EUT and the allowable work area are nearly the same. In this case, special attention should be paid to defining and documenting the test layout in the test setup. The EUT should be installed so that it operates under typical loads and other conditions in the vehicle to achieve maximum emissions. These operating conditions should be defined in the test plan to ensure that the supplier and customer can perform the same
Note: Different orthogonal orientations of the EUT may result in different measured electromagnetic energy levels
The positive power lead should be equipped with an RF filter at the TEM cell input. An artificial mains network should be used as this filter. The artificial mains network should be connected directly to the TEM cell and screwed tight so that the negative power lead is grounded at the connector panel. The RF sampling port of the artificial power network should be terminated with a 50 ohm load.
All sensor and actuator wires of the EUT shall be connected to the peripheral interface, which simulates the operation in the vehicle.
This interface should be connected directly to the BNC panel. The performance of the filter depends on minimizing the effect of the TEM's outdoor wiring, and the low-pass filter should be within the frequency range of the signal desired by the EUT. If no other configuration is specified in the test plan, the filter should behave like an artificial network with an impedance of 50 ohms to eliminate the effects of its length and placement. If using a BNC connector panel, the wiring inside the connector panel should be as short as possible, via 50 ohm coax. The shielding layer (outer conductor) of the cable should be grounded at both ends. Repeat measurements should be made using the same RF port of the TEM cell, with the other port terminated with a 50 ohm impedance.
5 Component/module radiated interference limits - TEM cell method
The level classes used (as a function of the frequency band) should be agreed upon by the vehicle manufacturer and the component supplier with recommended limits for component radiated disturbances.
Multiple limit value classes are defined because the mounting position, body structure and wiring harness design can affect the coupling of radio interference to the vehicle radio.
Table 1 Radiated Interference Limits - TEM Cell Method
6 TEM cell design
The dimensions of the TEM cell are shown in Figure 7a and Figure 7b and are given in Table 2; dimensions are in millimeters - not to scale
Figure 7a Horizontal section at the partition
Figure 7b vertical section of the partition
1 Allowable working area: 0.33 W, 0.60 L
2 access doors
3 dielectric brackets
Figure 7: TEM cell
Table 2 shows the dimensions for constructing TEM cells with specific upper frequency limits
Note: The TEM cell shown in the picture is a typical cell for automotive component testing. For integrated circuit testing, even smaller TEM cells can be used to test frequencies up to and above 1 GHZ
Table 2 - Dimensions of TEM cell
6 Relevant test standards for radiation emission TEM chamber method
Ø CISPR25
3. TEM cell method for radiation immunity of components/modules
1 Radiation immunity equipment configuration
Figure 8 shows an example of a TEM cell test setup. The TEM cell has high resonance in the region above the suggested upper frequency limit. A low-pass filter can be installed with an attenuation of at least 60 db at frequencies above 1.5 times the cutoff frequency of the TEM cell (e.g., 200 MHz TEM cell: 60 db for frequencies above 300 MHZ) to avoid resonance.
1 signal generator
2 broadband amplifiers
3 low-pass filters (optional)
4 dual directional couplers (minimum 30 dB decoupling ratio)
5 RF Power Meter
6 Peripherals
7 measured object
8 dielectric support
9 Low Pass Filter/Connector Panel
10 couplers
11 high power loads (50 ohms)
12 controllers
13 TEM cells
a forward (forward power)
b reverse (reflected power)
c output power
Figure 8 - Example of TEM cell radiation immunity configuration
2 Radiation Immunity TEM Cell Test Method
Test field strength calibration should be performed at all test frequencies using an empty TEM cell before the test field strength is applied to the DUT and wiring harness. This will determine the test forward power level (test) used during DUT testing. During the measurement, the door of the TEM chamber should always be closed. Unused connectors should be shielded from emitting radiation with a continuous RF signal at each test frequency. Use formula (1) to calculate the net power required to achieve the test electric field strength; apply the given forward power (see Figure 2), record the reflected power and calculate the corresponding net power, and record this test forward power level as positive direction, the test field strength adjusts the forward power level until the net power is obtained
here
E is the electric field value in volts/meter;
Z is the characteristic impedance of the TEM cell in ohms (usually 50 ohms).
Pnet is the net input power (Pnet=P forward-P reverse), the unit is watts;
d is the distance between the floor and the TEM chamber partition in meters
The test forward power level established during field calibration in an empty TEM chamber (at the required test field strength) will be the leveling parameter used during DUT testing. During DUT testing, the test power required to achieve the test field strength level should be maintained at positive (test) 0/+1 dB.
3 Functional Performance Status Classification (FPSC)
This article gives examples of test severity levels that should be used in accordance with the principles of the Functional Performance State Classification (FPSC) as described in ISO 11452-1 Test Severity Level Classification. The recommended minimum and maximum severity levels are shown in Table 3 below.
Table 3 - Example of test severity levels (infield)
4 Relevant test standards for radiation immunity TEM chamber method
Ø ISO 11452-3
Ø SAE J1113-24
Ø IEC 61000-4-20