TEM-cell Test Guide for Radiated Emissions and Radiated Immunity

Introduction：

When you are looking for a test instrument for EMC radiated emissions and radiated immunity testing, nothing compares to the capabilities of the TEM cell.

The TEM cell (or GTEM) provides a low-cost method of measuring product emission and helps to subject products to radiation fields.

In some cases, the FCC will even accept GTEM results for device authorization. Continue reading below to learn how this works.

What is a TEM-cell?

"TEM" stands for "Transverse Electromagnetic Mode". What does that mean? Well, a transverse mode of electromagnetic radiation is a specific field pattern radiated in a plane perpendicular to (i.e., transverse to) the direction of propagation of the radiation.A TEM cell is a rectangular coaxial transmission line that tapers at each end to form a geometry specifically designed to create a TEM pattern over a given frequency range (typically DC to several hundred MHz).

The useful frequency range of a TEM cell is determined by its geometry, but generally follows the following rule: the larger the TEM, the lower the upper frequency range available. Above a certain cutoff frequency, higher order modes are created (as shown below), which can destabilize all test volumes throughout the test volume.

Reasons for frequency bandwidth limitation of TEM

"Perhaps the easiest way to explain the TEM is through what it does: the TEM allows you to measure the radiated field strength of a product and subject the product to a given field strength."

TEM-cell main structure:

TEM cell main structure

The main structure of a TEM cell is shown above (technically a "triple-plate line" or "triple-plate open TEM cell" because it includes an additional grounding plate at the top), and they include:

RF connector: Used to connect to a coaxial cable which in turn connects to a spectrum analyzer or signal generator/power amplifier, depending on whether it is used for transmit or immunity testing.
EXTERNAL SHIELD: The shield connected to the supplied coaxial cable. On the three-plate model shown, there are two ground plates. Technically, the TEM-cell requires only one grounding plate.
DC Module: Used to protect the analyzer's sensitive inputs in the unlikely event that a DC voltage is accidentally applied to the spacer.
Spacer: center conductor.
50 Ohm Load: This is the termination of the transmission line. As with any transmission line, if it is not properly terminated (e.g., a larger mismatched load), it will receive signal reflections.

Detailed construction of the TEM cell:

Here are the details of how the TEM cell was made. To do this, I disassembled the ESDEMC EM601-6 and TEKBOX TBTC1 cell and took a few photos:

Detailed structure of TEM cell

This design makes the electric field lines between the diaphragm and the outer plate fairly uniform.

A "test volume" is created between the outer shield and the spacer where the product under test is placed. Within this test volume, the measured values are fairly uniform, only a few dB.

The data show that the measured value of the 500 MHz test signal varies from about -4 dB to + 5 dB within a volume of 12 cm x 12 cm x 9 cm. In the EMC field, this is quite stable.

Characterization of field uniformity in test volumes

What is the TEM-cell for?

In the following section, I'll dive into all the various use cases for the TEM cell.

Radiated emission pre-consistency testing with TEM-cells

One of the primary functions of a TEM cell is to test the radiation field emitted from the DUT (Device Under Test). Engineers are always looking for an illusory way to determine if their product passes or fails in the test lab. Near-field probes cannot do this because it is impossible to accurately extrapolate from near-field to out-field without phase information.

Waves traveling through the TEM cell have a free-space impedance of about 377 Ω (up to the upper frequency), which happens to be the impedance of a far-field plane wave propagating in free space. In an OATS (Open Area Test Site) or a semi-anechoic chamber, the measurement antennas are placed at 3 m or 10 m intervals, which corresponds to being in the far field at the corresponding frequencies of 100 MHz and 30 MHz, respectively. Measurements with the TEM-cell are therefore closely related to measurements made in a full-scale test laboratory.

Incidentally, this 377Ω free-space impedance describes the relationship between the electrical amplitude (V / m) and the magnetic field (A/ m), similar to how a 377Ω resistor correlates voltage (V) to current (I) according to Ohm's Law. For fully-compatible radiated emission tests, the measurement transducer is usually to the electric field, a biconical or horn-shaped antenna, which are both sensitive to the electric field. Due to this free-space impedance, we can measure the magnetic field of the EUT equally, but as far as practicalities such as antenna size and shape are concerned, it is more convenient to use an electric field antenna.

Radiation emission test set

TEM-cell test setup for radiated emission testing

One of the advantages of the TEM-cell is its simplicity. Connect one port to the spectrum analyzer via the optional DC module, connect the other port to the terminating resistor, and place the product between the spacer and the external conductor to start the measurement.

Tune your spectrum analyzer to the frequency range of interest and you'll soon find out if there are any suspicious emissions.

environmental noise

If you are using an open TEM cell, then you may also see ambient noise signals such as FM, Wi-Fi and cellular radio transmissions. Open area test sites also have this problem. There are a number of techniques that can be used to determine if the emissions are "ambient" or from the EUT:

Turn off the EUT. if the emission disappears. from the device under test.

Move/rotate the EUT to see if the amplitude of the firing increases or decreases.

Reduce the span of the analyzer so that it focuses on only one peak (eh span = 1 MHz) and reduce the resolution bandwidth (RBW) to 10 kHz or less. This will usually show emission hidden behind the ambient at higher RBW.

Background noise of TEM-cells

A common problem with TEM cells is that the emission intensity of the EUT may not be strong enough to be detected above the noise floor shown on the spectrum analyzer screen. There are a number of options available to increase the measurement sensitivity:

If the size of your EUT allows it, use a smaller (more sensitive) TEM.

Insert a preamplifier between the TEM and the spectrum analyzer. Common preamplifier gains are 20 dB or 40 dB. higher gains risk saturation or damage to the spectrum analyzer input stage.

Reduces internal fill attenuation in the spectrum analyzer to zero.
Reduce the RBW setting of the spectrum analyzer.
Use a masked TEM (see the section below for more information).

Between all these methods, you should be able to see all the narrowband and broadband emissions of interest.

Limitations of TEM for radiometric measurements

It would be fantastic if the TEM-cell could tell us exactly whether or not our products comply with regulatory limits (e.g., FCC radiated emission limits).

There are a few issues that make this a challenge (but not impossible!) :

Often, the radiating component that causes RE failures (especially below a few hundred MHz) is the cable connected to the EUT. In small TEMs and even larger TEMs, the cable layout plays a big role in the measurement amplitude. This is why I recommend pairing a TEM cell with a current probe.

Emission tends to be directional (as opposed to isotropic), which means that the orientation of the EUT in the TEM can greatly affect the measurement. In some GTEMs, manufacturers try to solve this problem by automating the measurement process. They perform measurements of the EUT in the X, Y, and Z directions and combine the measurements to produce a value that they claim correlates well with far-field measurements. For more detailed information on GTEM and correlation, see below.

Radiation Immunity Testing with TEM-cells

Another common use case for TEM-cells is radiated immunity testing:

The signal generator and power amplifier are connected to one port and the 50Ω terminating resistor is again placed at the second port.

TEM-cell test setup for radiated immunity

In this configuration, a very stable magnetic field is established between the spacer and the outer cover.

The electric field in volts per meter (V / m) between the spacing of the TEMs and the shield is given by the following simple equation:

E = V / d where V is the RMS voltage of the applied signal and d is the distance between the spacer and the lower (or upper) conductive layer. This is based on the simplifying assumption that the electric field is perfectly uniform (i.e., uniformly distributed).

A more practical formula is:

E = V * Cor / d where Cor is a correction factor for the average field strength over the volume of the EUT, which is derived by analyzing the field distribution over the cell cross-section.

In particular, for the TekBox TBTC1, the following equations can be used with sufficient accuracy, assuming that the EUT is placed in the center of the cuvette and midway between the base plate and the spacer.

d = 5 cm E = (√(P * 50Ω))* 20
d = 10 cm E = (√(P * 50Ω))* 10
d = 15 cm E = (√(P * 50Ω))* 6.66

This produces the following field strength:

Maximum electric field strength of TEM cell at different input powers

What you will notice here is that relatively high field strengths can be produced using relatively small amplifiers.

44 V / m can be generated in TBTC1 with only 0.1W.

The equivalent test setup in a full compliance test laboratory is shown below (according to IEC 61000-4-3):

IEC 61000-4-3 Radiated Immunity Test Configuration Chart

To generate 44 V/m in a test lab, you need:

Anechoic chamber ($ 500k - $ 1m)

2 or 3 high power antennas

High power RF amplifier (> $ 50k)

In comparison, the TEM cell looks pretty good! This is why many manufacturers invest in TEM cells or GTEM for radiation immuno pre-consistency testing and troubleshooting.

Some even use TEM cells for full conformance testing. More below.

EMI Troubleshooting with TEM-cells

The TEM cell is particularly useful for comparing configurations. If your product fails in an EMC test lab and you want to make modifications and test the effects of the modifications, the TEM will allow you to do this in your office without having to use an expensive test lab.

While the TEM doesn't allow you to precisely isolate where radiation is being generated on the board, a set of low-cost near-field probes can be used to solve this problem. The TEM comes into play when you want to quantify the effects of PCB changes

It is well known that the same components from different suppliers can have very different EMI performance. This is one of the reasons why I really recommend sourcing some models and then evaluating their EMI performance in a TEM cell. You will be able to see how they compare to each other very soon.

This technique is useful for evaluating the following components:

liquid crystal display

IC Module

power module

...... just to name a few. I have seen components with the same specs on paper vary in EMI levels of +/- 40dB. This is a critical step.

Another "catch" to be aware of is that some unethical vendors will remove the "expensive" filtering components after evaluating the initial gold equipment provided to you. In this case, the EMI performance of your first batch of product runs may be completely different from the pre-consistency tests you performed.

Benchmarking production cell assemblies and previously evaluated assemblies using a TEM cell will help address these issues.

Monitor ongoing compliance with TEM cell after component replacement

At last year's EMC Symposium, I had the opportunity to ask the EMC instructor what was the biggest trend in EMC that he had noticed recently. His response was not what you might expect.

One of the biggest challenges he sees is that IC manufacturers are shrinking chip sizes to reduce costs. In the process of shrinking the chip, the slew rate decreases and the peak RF current demand increases, so emissions increase. Little or no mention is made of these die swaps, but this can change a product from compliant to non-compliant without the manufacturer knowing.

Testing cell samples from new production lots and comparing them to baseline results will again help to resolve such issues.

RF transmitter measurements using a (G)TEM cell

TEM cell of another structure

Another useful feature of the TEM cell is to allow fundamental and harmonic emission testing of intentional transmitters.

In this excellent application note from Nokia Engineers in Battle es, Stephen Clay describes his efforts at Nokia to correlate measurements in GTEM with measurements taken in the indoor and OATS of cell phones.

He found that he could get good correlations considering setting up the test setup correctly and measuring through all 4 directions.

Isotropic magnetic field probe calibration using TEM cell

One of the main benefits of TEM is that it produces a very stable and mathematically predictable field strength between the spacer and the outer plate.

This property can be used to accurately calibrate isotropic field probes over a wide range of frequencies.A method for accomplishing this is described by K. Matloubi in his paper " Methods for Calibrating Isotropic Electric Field Probes".

After calibration, the field can, somewhat ironically, be calibrated using a field probe to perform radiated immunity tests in a semi-anechoic chamber.

Shielding performance testing with TEM-cells

Measurement of the shielding effect of a material using two TEM cells

Using the rare dual TEM cell , it is possible to generate a known field in one TEM and accurately measure the attenuation field in the other TEM.

The shielding effect of a material can be accurately characterized by placing the material under test in a hole between two TEMs.

Different types of TEM-cells

As you may have noticed by now, there are many different types of TEM cells . Read below to learn about the advantages and disadvantages of each use.

Open TEM cell

Open TEM-cell

An open TEM cell like the one shown above is a good low-cost option.

Pros:

inexpensive
Lightweight and easy to store
Large EUTs can be slid in from the side
If the device under test is long, it can be tested at multiple points along its length
No filters required for cable entry/exit TEM

Drawbacks:

Open to receive ambient noise
Radiation immunity testing may be illegal unless used in a shielded room or tentThe
Depending on the TEM size, the upper frequency limit is limited to a few hundred MHz

Enclosed (shielded) TEM cell

Pros:

Reduced/eliminated narrowband ambient noise measurements
Radiation immunity testing can be accomplished without worrying about legal issues
Reduced noise floor

Drawbacks:

EUT size limited by door size
Getting the cable out of the TEM can be a challenge. Unfiltered cables transmit noise inside and outside the chamber, so custom filter panels are often required
It is also possible to use an open TEM in a shielded enclosure. does not have to be a shielded chamber; it can be a smaller shielded tent structure as shown below.

screened tent

The graph below shows the situation with (pink) and without (yellow) the shielding tent. As you can see, the ambient noise disappears and the total background noise is reduced by 10-13 dB. this makes it easier to tell what the product you are testing is emitting and potentially exposing radiation that could have been masked in the test setup background noise.

Unshielded (yellow) vs. Shielded (pink) Measurement Chart

Incidentally, for the current probe measurements that may be required for automotive module testing, a shielded tent such as this has a similar effect on the noise floor. For the low limit lines specified in some standards, it can be a real challenge to get enough margin between the noise floor and the limit lines. A shielded chamber or hot spot can help with this.

GTEM

If you can't afford a semi-anechoic chamber, but still want to do your emission and immunity testing in-house, then GTEM may be the next best thing.

I have written about GTEM in the past, such as the EMC Pre-Conformance Testing Guide and the Emissions Pre-Compliance chapter of the EMC Testing Primer. They are very useful tools, but they have their limitations. Read some of the pros and cons below.

GTEM cell

Pros:

Can hold a fairly large EUT
Can be used for "full compliance" emission and immunity testing in some cases (see section below)
Good correlation with chamber / OAT measurements if the "Boss Manipulator" is installed
Depending on the model, the measurement range can be extended to about 20 GHz.

Drawbacks:

EUT size limited by door size
Cable management can be a real problem
Filter panel required to route all cables into and out of the GTEM
Poor low frequency response reduces correlation with OATS measurements below a few hundred MHz

AR Cell

One variation of the GTEM is Amplifier Research's AR Cell. they are no longer manufactured, but you may see them in the wild.

AR CELL

These are a hybrid between a semi-anechoic chamber and a TEM cell.The interior of the ARcell enclosure is lined with RF-absorbing material to create a self-contained semi-anechoic enclosure. It includes two field transmitter and receiver devices. An internal log-periodic antenna acts as a transmit/receive device at higher frequencies, while an optional bowtie antenna can be used at lower frequencies. It allows vertical and horizontal polarization.

Pros:

Evolutionary design-create localized FACs inside the cell
Field uniformity by limiting reflections using absorbers
Ideal for radiated immunity testing

Drawbacks:

Emission accuracy of general limits
Absorbers reduce RF input efficiency and produce the required field strength (typically up to 10 V/m at 80% modulation) requiring more power

(sth. or sb) else

There have been other variations of the TEM cell / GTEM over the years. These include LaplaCell, Eurotem and WTEM. each with its own pros and cons.

Correlation of TEM cell with OATS / chamber

One of the most common questions I get is whether or not you can use a near-field probe or TEM cell to determine if your product passes or fails in an EMC test lab. In the use case described above, this use case is called "Radiation Compliance Testing".

Fortunately, there is some good data on the correlation between GTEM cell and OAT and chamber measurements.

The most extensive paper I found on the subject was by Angela Nothofer and Martin Alexander of the UK's National Physical Laboratory and Didier Bozec, Andy Marvin and Les McCormack of York EMC Services, " Using the GTEM cell for EMC measurements".

In the graphs below this article, you can see that the correlations measured from 32 MHz to 6 GHz are quite good in the GTEM using the CNE reference source. Note that to get these results they had to pay extra attention to the measurement method.

Comparison of GTEM and Open Field OATS Tests

Two test modes for GTEM

One of the main challenges when using TEM-cells for radiation pre-consistency testing is that the main source of radiation is usually not the board, but the cables connected to the board. This is a problem at lower frequencies (below a few hundred megahertz) and becomes less of a problem as the frequency increases. As the frequency increases, the primary mode of emission switches from a common mode signal to a differential mode signal originating from the circuit board itself.

FCC Regulations for GTEM-cells

The correlation between GTEM results and OATS / chamber results is so good that in some cases the FCC will accept GTEM cell data.

In Notice GTEM PN 40830, issued December 2, 1993, the FCC agreed to accept radiated emissions test data under limited conditions for the purposes of Part 15 equipment authorization. These conditions are:

" 1. Acceptable comparative measurement data must be archived with the Sampling and Measurement Department and appropriately analyzed to demonstrate equivalence to an open area test site certified to meet the requirements of C63.4-1992. Comparative data must be provided for each generic type of equipment under test (equipment with similar functionality and physical configuration, e.g., handheld transmitters, laptops, etc.) for future acceptance of GTEM data. Statistical analysis must support the validity of the calculated correlation coefficients.

2. Acceptance of GTEM data to demonstrate compliance with radiation limits will be limited to the general type of equipment under test that has been previously demonstrated to be equivalent to airspace testing. Any wiring, whether external or interconnecting, must be aligned and terminated in the same manner as the equivalent demonstration performed by the previous submitter.

3. In the event of disagreement, final radiated emission tests performed at open area test sites meeting the requirements of ANSI C63.4-1992, Sections 5.4.1 and 5.4.6, shall take precedence."

Then, in bulletin GTEM PN 54796, issued on July 12, 1995, they again expanded the acceptance of GTEM results under limited conditions to include parts of the rule other than part 15

In accordance with the guidance on the test procedures and data reports you are required to use, the FCC specifies " ANSI C63.4-2014 Attachment F - Test Procedures for Emission Tests in TEM Waveguides (30 MHz to 1 GHz)" and "Attachment G -Conversion of OATS Samples to TEM Waveguide Validation Spreadsheet". It is recommended that you purchase this standard if you wish to explore this avenue of authorization.

Buy TEM-cell:

Our company represents and sells many brands of TEM-cell, GTEM and other test instruments, we can also according to customer demand together with the spectrometer and receiver to customers for radiation emission and radiation immunity testing. Click on the product links above for more information.

TEM cell
GTEM
spectrum analyzer

Conclusion:

As you can see, the TEM cell is a versatile tool for EMC engineers to perform radiated emission and radiated immunity testing for any company. For emission pre-consistency testing, immunity pre-consistency testing, EMC / EMI troubleshooting, component evaluation and even FCC testing, the TEM cell is an excellent test instrument that can help you solve many problems.

Originally Posted by Andy Eddy

Andy Eadie is a former Senior Hardware Design Engineer and the former owner of an EMC test lab. Since the age of 4, he has developed a strange fascination with magnets, and he has been publishing articles, eBooks and online courses.