Updates on the New Release of IEC 61000-4-3 Edition 3
By Jason Smith, Sr. Applications Engineer, AR Worldwide

The IEC 61000-4-3 has been used for many years as the basic test standard for testing to radiated, electromagnetic field immunity testing in order to satisfy one of many European Union requirements for the CE mark. This standard is usually used in conjunction with a product standard that will specify this standard and other test standards detailing the requirements the product must meet. The product standard may give additional guidance on how this test standard is used, including test level severity and changes to procedure.

The object of this test standard is to establish a common reference to radiated RF immunity caused by any source. Electronic products need to be designed and tested to have immunity to these sources. Radiated RF sources can radiate from many sources, such as other electronic devices, electric motors and intentional transmitters such as walkie-talkies and cell phones. The introduction of more wireless devices in the past couple of years has increased the need for this testing, not only to satisfy governmental requirements but also to increase product reliability which increases customer satisfaction.

IEC 61000-4-3: Electromagnetic compatibility (EMC) - Part 4-3: Testing and Measurement Techniques - radiated, radio-frequency, electromagnetic field immunity test.

Edition 3 has been approved and released ahead of schedule. Table 1 shows the status.

Major Changes in IEC 61000-4-3 Edition 3.0:
• New harmonic distortion requirement of test setup: better than 6dBc
• New linearity check to make sure the RF amplifier is not operating in compression
• New extension of frequency range up to 6GHz
• New test table material requirement

The above changes could have a big effect on some facilities and test equipment.

Harmonic Distortion
Harmonic distortion is the difference in dB between the fundamental and harmonic of the fundamental. The new standard calls for a harmonic distortion of better than -6dBc for the test setup. This means all harmonics need to be 6dB below the fundamental out of the transmitting antenna. This could become a problem when using RF amplifiers in compression or when using Traveling Wave Tube (TWT) amplifiers. TWT amplifiers have been used historically for testing above 1GHz when high power is required and can offer significant cost/performance benefits at very high power levels. Since the introduction of solid-state amplifiers in this frequency range, many of the limitations of TWT amplifiers have been overcome, and at lower power levels solid-state amps are readily available. TWT amplifiers can still be used for high field testing but care must be taken in order to satisfy the standard. Some TWT amplifiers include methods for harmonic reduction by combining tubes or switching in filters. For most other TWT amplifiers, RF filters can be attached externally to block out these unwanted harmonics. Above is an example of the harmonic content of a solid-state and a TWT amplifier (20 watt rated). The solid-state amplifier has an excellent level of -24dBc, while the TWT amplifier's level is only -0.8dBc.

The harmonic of the TWT will contribute to the calibration level since a broad band RF field probe is used and it can not distinguish between a wanted and unwanted signal. In addition, antenna gain usually increases over its operating band. The antenna gain can be as much as 5dB higher at the harmonic. In the above case, the -0.8dBc harmonic distortion of the TWT will result in a much higher field level at the harmonic then at the fundamental. These harmonics will result in a significant error in field level. RF filters will be needed. The consequence of using filters will be some loss in power and productivity to switch these filters in and out. The amplifier used will dictate what frequency range/s the filter will need to cover, or if multiple filters will be needed. Most solid-state amplifiers will not need filtering for this reason.

High harmonic content can also have an unwanted adverse effect on the Equipment Under Test (EUT). The harmonic could be at a frequency that causes the EUT to fail. Since the test personnel are testing at the fundamental frequency, they will mark this frequency as the failure, which is not the case. It is quite possible that this harmonic may be outside the intended test frequency range, so therefore should not even be part of the test. From the EUT manufacturer's point of view, harmonics are very much unwanted since this signal can cause failures and is not part of the test.

If we work backwards, we can estimate an acceptable amplifier harmonic content rating:

Therefore, a harmonic content for the amplifiers better than -14dBc will be more then acceptable.

This allows for a safer harmonic content requirement for the amplifier, guaranteeing an acceptable harmonic level during testing. Keep in mind not to run the amplifier into compression and use filters when necessary to reach this requirment.

Linearity Check
The Linear region of an amplifier is the power range in which there is a 1:1 ratio in input (dB) change to output (dB) change. As the amplifier starts to saturate, this will no longer be true. AR's solid-state amplifiers are specified at both a 1dB and a 3dB compression point. Below the 1dB compression point, the amplifier's response is referred to as linear. Above the 3dB compression point, the amplifier is in full compression.

There are two main reasons why this is important. Firstly, if amplifiers are running in compression, the output signal will be distorted. This means a sine wave will start to resemble a square wave. In the case of the IEC 61000-4-3, amplitude modulation (AM) will also be distorted, possibly causing different unrepeatable test results. Secondly, when an amplifier is running closer to saturation, the harmonic content will increase.

Figure 2 shows one of AR's solid state amplifiers and how AR finds its 1dB and 3dB compression points. Basically, the point where the input is increased by 10dB results in only a 9dB change on the output will be the 1 dB compression point (Orange triangle). The point where the input is increased by 10dB results in only a 7dB change on the output is the 3 dB compression point (green triangle).

The new specification calls for a check for a 2dB compression point (aqua triangle) while connected to the antenna. If the load (antenna) on the amplifier is a pure 50O, then amplifier manufacturers could easily specify this new 2dB compression point to be used as a reference when calculating your needs. But, since any antenna which is used during testing is not a pure 50O load but an unknown complex load, the compression point may very slightly. For this reason, it is best to size amplifiers based on the manufacturer's supplied 1dB compression point to allow for some margin of error. This will be a concern when using a TWT amplifier since they are normally not as linear as solid-state amplifiers. The 1dB compression point is usually around 50% of its rated power, therefore a 20 Watt TWT amplifier will have a 1dB rating of about 10 Watts. In the case of the 25S1G4A solid-state amplifier, it has a minimum output rating of 25Watts. The minimum 1dB compression rating is at about 20Watts. Figure 3 shows production data of a 25S1G4A amplifier and at 1.5GHz is producing 38 Watts with about a 1dB compression rating of 30 watts. It is always best to check the amplifier's specification or to contact the manufacturer directly for this information and assistance with the selection of the product that will meet your needs.

Increased Frequency Range
The frequency range increase from 2GHz up to 6GHz is directly in response to the use of more of the RF spectrum by the communication industries. In different countries and locations, the RF spectrum is being divided up depending on each country's laws. So, based on where the Equipment Under Test (EUT) is being used, not all frequency bands may need to be tested. In addition, not all communication standards use the same signal strengths. This is why this new test standard is leaving further definition up to forthcoming product standards. Product standards will specify what additional frequencies to cover in the communications bands: 800 to 960MHz and 1.4 to 6GHz. Product standards will also specify test levels which may not be consistent throughout the bands. The current 80 MHz to 1GHz should remain the same.

Chamber setups may have to change because of this higher frequency requirement. Many labs only have a ferrite lined chamber. This works well when testing less than 1GHz, but as frequency increases, it is difficult to meet the field uniformity requirements. This is because the ferrite is not very absorbent above 1GHz and will reflect the RF field. Annex C of IEC 61000-4-3 Ed. 3 explains this situation and gives good advice and options on correcting this problem. A fully lined anechoic chamber with ferrite and absorber material will work fine.

Test Table
The test table is now specified to be made of low permeable material. Rigid polystyrene is one material that is suggested. In the past, many labs have always used wood, which is fine when testing at lower frequencies. Now that testing can be as high as 6GHz, wood will start to have some unwanted properties. High frequencies will reflect, making it difficult to meet the uniform field requirement. The reflections will also make test results less repeatable.

General Test Tips
The above new requirements could require labs to upgrade and purchase new equipment. But some helpful hints may increase the likelihood of success.

If harmonic content is an issue:
» RF filters on the amplifier's output may fix your problem
• Make sure the additional losses of filters do not force the amplifier into saturation
• This will increase test time to switch filters in and out

If working in saturation:
» Reduce all RF losses in the system
• Use good low loss RF cabling and connectors
• Make sure all connections are torqued to specification
• Make sure all connectors are clean
• Shorten RF cabling (may require amplifier to be moved closer to the antenna)

» Use a different RF antenna
• Higher gain antennas will require less power
• Keep in mind narrow beam width may not cover the full window of 1.5m x1.5m uniform field calibration requirement - Calibration to smaller window is allowed above 1GHz
• Horn antenna will direct the energy forward better then Log antennas - Resulting in better field performance

» Move antenna in closer, no less than 1 meter

If the above considerations are taken into consideration and the requirements can still not be met, a new amplifier may be needed.

Conclusion
The increased frequency range of this standard has brought some common test problems to light as to their contribution to test error. Continuing efforts must be taken to maintain a consistently repeatable test. Both harmonic distortion and amplifier linearity are issues that until now have been overlooked by this standard. All RF amplifiers can run in compression and produce harmonics. If the amplifier is running in compression and if the antenna and cabling can not be improved upon, a higher power amplifier will be needed. If harmonic content becomes an issue, RF filters will be needed to block out these harmonics. With AR's solid-state amplifiers, filters will not be needed. It is also a good idea to ask manufacturers for examples of test data taken from production units. This data can aid in product selection and give a better level of confidence of the manufacturers ability to meet their published specification. Quality of testing and repeatability should be the goal of every EMC test lab. Repeatability is the important goal of these international test standards.

Note: This article should not be used in conjunction with or instead of the official released publication. It is only intended to give you guidance and information on the changes. Always refer to the latest standards.

AR WORLDWIDE
www.ar-worldwide.com
TXTLINX.COM73
Email this article to a friend!