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The regulatory bodies want to make sure that an item with an internal combustion engine does not cause unwanted interference with TV and radio reception when it drives past or is used nearby a residence or business.
CISPR 25 is not typically used for regulatory purposes, it is commonly used by the vehicle manufacturers to assure good performance of receivers mounted on-board the vehicle.
If the radio mounted in the vehicle, boat or other device does not perform reliably, then consumer satisfaction and ultimately product sales could suffer. Both CISPR 12 and CISPR 25 deal with automobiles vehicles which operate on land powered by internal combustion engines, boats vehicles which operate on the surface of water powered by internal combustion engines, and devices powered by internal combustion engines but not necessarily for the transport of people.
This last category includes compressors, chainsaws, garden equipment, etc. CISPR 12 would apply to all of these devices since they could affect the performance of nearby off-board receivers.
As an example, a chainsaw with an internal combustion engine but with no on-board receivers would need to meet the requirements of CISPR 12, but CISPR 25 would not apply to this chainsaw since it does not utilize any on-board receivers. CISPR 12 radiated emissions measurements are made at either 3 meter or 10 meter test distances.
Measurements for boats can also be made on the water. The specification currently does not provide a method to achieve this correlation. CISPR 25 has two parts. One part deals with a full vehicle or system test in which the antennas mounted on the vehicle are used to sense the noise generated by the different electric and electronic systems mounted on the same vehicle.
This test shows how much noise generated by the vehicle will be introduced into the radios antenna port sort of a self-immunity test. The other section of the standard deals with conducted and radiated measurements of vehicle components and modules.
In this article, we are going to concentrate on the module radiated emissions test section of CISPR 25, and only briefly highlight some of the additions needed to support electric vehicles. CISPR 25 states that the electromagnetic noise level in the test area has to be 6 dB lower than the lowest level being measured.
An RF shielded room is typically used to keep RF signals from the external environment out of the test area so that the equipment under test EUT remains the dominant source of any radiated interference.
Although the shielded room is too small to support resonant modes at low frequencies, the number of modes increases with frequency above the cut off of the chamber.
When these resonant modes appear, they can add significant error to the measurements. To reduce these errors, the shielded room covered with RF absorber material on its ceiling and interior walls, greatly supresses internal reflections so that the dominant coupling path is between the EUT and measurement antenna. One beneficial consequence of the low measurement frequency is the fact that the chamber sizes are electrically small at these low frequencies, so no significant resonant behaviour appears.
The standard therefore concentrates on absorber performance at 70 MHz and above. The standard requires that the absorber used must have better than -6 dB absorption at normal incidence.
To achieve these levels, there are several types of absorber technology on the market today. One of the most efficient and cost effective is a polystyrene based absorber that combines a high-performance ferrite tile with a polystyrene EMC absorber, having 60cm x 60cm base and 60cm height. The main absorber substrate is based on expanded polystyrene EPS , which is volumetrically loaded with lossy materials, and environmentally friendly fire retardants. Advanced uniform loading in the manufacturing process results in superior RF performance an excellent absorption uniformity.
The closed cell structure of this type of absorber makes it suitable for use even in high humidity environments. These features all contribute to providing for a better controlled and predictable chamber test environment.
But, without the benefit of the matching ferrite material used in the hybrid, the polyurethane only absorber suffers from reduced low frequency performance. The cables are routed in a cable harness that is positioned along the front edge of the bench. The cable harness itself is a significant component of the EUT and is the main component illuminated by the measurement antenna since at lower frequencies frequencies for which the device under test is electrically small the main coupling to radiated fields will occur through the cables feeding the device.
This same procedure is used in MIL STD  and in ISO  and as shown in the illustration, a line impedance stabilization network is used to provide a defined impedance for the power to the device. The ground plane bench must extend all the way to the shield and in most cases, it is grounded to the wall of the shielded room. The standard, however, does permit the bench to be grounded to the floor as an alternative. The minimum overall dimensions of the compliant chamber are determined by a series of dimensional relationships based primarily on the size of the test bench.
For chambers that will also be used for e-motor testing, the motor is also be part of the EUT. In some cases the motor is supported on a separate structure adjacent to the test bench for mechanical reasons. In this case it still needs to be connected to the ground plane so in effect it will be an extension of the ground plane bench and subject to the minimum distances as defined in the standard.
The first and most critical is the test distance where emissions are to be measured at a minimum distance of 1 m from the cable harness to the antenna. The other rule states that no part of the antenna can be closer than 1 m away from the tips of the absorbing material.
These rules and recommended antennas define the length and height of the chamber. The 1 m distance to the cable harness is measured from the axis of the antenna elements for the monopole rod and the biconical antenna. For the log periodic dipole array LPDA , the distance is measured from the tip of the antenna. Finally, for the horn antennas the distance is measured from the front face or aperture plane of the antenna.
The longest antenna is the LPDA. In addition to the 1 m test distance and the 1 m for the antenna length, we have a 1 m clearance from the back of the antenna to the tips of the absorber. The longest antenna is usually the active rod monopole. The monopole is used with an extremely electrically small ground plane.
And, as we will see in the next section of this article, this chamber will also meet the requirements of ISO Furthermore, since this is a shielded environment, most of the tests defined in standards requiring a shielded room can be performed inside the chamber described in the present section.
For low frequencies, an active rod monopole antenna is preferred. At frequencies between 30 MHz and MHz, a typical biconical antenna is the recommended antenna. The next revision of the standard will contain an annex Annex J which provides methods to validate the performance of an ALSE used for component level radiated emission tests. Currently, two methods one method based upon reference measurements and another method based upon modelling are being proposed for the ALSE validation tests. The user can select either method to evaluate the performance of the chamber as the work done by the committee has shown that either method provides similar results.
As mentioned at the beginning of the article, CISPR 25 also covers the measurement of emissions received by a vehicle antenna for a whole vehicle setup. The committee found that special testing and limits are required for the testing of these electric driven vehicles and their components.
These vehicles represent a special case since there are high currents and voltages involved not only in normal operation but also during charging cycles. There will be more detailed information on the measurement setups to be used for EV and HEV measurements under different connection and charging scenarios. The testing adds new conditions for when the vehicle is not being driven, but connected to the mains or a charging station. This standard, like many automotive, military and aerospace standards, calls for moderately high fields to be generated.
At frequencies below MHz, antennas get physically larger and also less efficient. For frequencies below MHz, the standard recommends the methods stated in parts 4, 3, and 5 of the ISO standard. Those sections describe the bulk current injection, TEM and stripline test methods. These other methods are far more efficient and economical to test for immunity to high fields. Severity Level.
ISO 11452-5: Stripline
ISO 11452-5 PDF