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CISPR 25: “Vehicles, boats and internal combustion engines - Radio disturbance characteristics - Limits and methods of measurement for the protection of on-board receivers”
CISPR 25 is an automotive standard that covers the main emissions testing methods needed to ensure self-compatibility between a vehicle and its own on-board receivers..
CISPR 25 governs emissions related to automotive components, both for ground vehicles and boats. It includes both component level and vehicle level tests. The current version is the 5th edition published in 2021, and you can purchase it here. Some of the component level test methods in CISPR 25 are very similar to comparable methods in MIL-STD-461, but they are not at all similar to emissions testing done for the FCC via ANSI C63.4. (You can check out one of my presentations that dives pretty deep into that comparison.) And obviously MIL-STD-461 does not have an equivalent of the full vehicle testing found in CISPR 25.
More than anything, CISPR 25 is aimed at self compatibility. Historically this meant limiting emissions that would interfere with onboard radios (AM/FM/DAB) so as to avoid customer complaints. Today with vastly more complex vehicle electronics and the HV components of EVs, there are a lot more “victims” that can be affected by emissions than just AM/FM radio. [Aside: I once worked on troubleshooting an EV where a CISPR 25-noncompliant inverter interfered with the vehicle’s twisted/shielded CAN lines to the extent that whenever the driver stepped on the “gas” pedal, the battery control module shut down, requiring a full restart. In that case the solution was improving the shield terminations on the CAN lines.]
In general, vehicle level tests are done in four possible modes: key on, engine off; internal combustion engine (ICE) in driving mode (spinning wheels on a dyno); EV in charging mode, and EV in driving mode. (A plugin hybrid electric vehicle would need to test in all four modes.) For testing on the full vehicle, you need to disconnect each antenna from its receiving radio module, and feed the antenna coax into the measurement receiver, often through an impedance matching unit.
At the component level, CISPR 25 includes both conducted and radiated emissions tests. Conducted emissions can be measured via voltage measured from an artificial network (AN, equivalent to a LISN in the aerospace/defense world), or via a current probe measured at a minimum of two locations along the cabling. For radiated emissions the main test method is conducted in a semi-anechoic chamber, with an informative Annex F describing a stripline method. It currently does not provide for a reverb chamber method.
TIP:
Pay close attention to the flowchart in Figure 1 of CISPR 25. Because of the different limit lines for different detectors(peak, quasi peak, and average) and how they apply to narrowband vs. broadband noise sources, there are a lot more steps before saying something definitively “passes” or “fails” than in a method like MIL-STD-461 RE102.
TIP:
CISPR 25 is a critical standard for the HV components of electric vehicles as well as the traditional 12 Vdc systems. Take this as one consultant’s experience, but every EV on which I’ve done troubleshooting had problems with an HV component that failed CISPR 25 testing (particularly inverters and DC/DC converters). I have not yet had to troubleshoot on an EV where every component passed CISPR 25. It is a major challenge to design an HV inverter that passes CISPR 25 limits, but it can be done–and it’s likely worth investing the extra time to design a compliant system, or the extra money to buy one. You can also see my presentation on Noise Sources in EVs for more.
TIP:
Getting to the audio head unit (radio) to disconnect the AM/FM radio for vehicle level testing can be painful, and the sweeps can take a long time, especially if there are a lot of frequencies where quasi peak measurements are needed. CISPR 25 is not a regulatory test, so judgment of compliance is up to the manufacturer. It may make more sense to run the vehicle and tune through stations looking for audible problem areas, then only run the CISPR 25 sweeps on frequency ranges where there are issues. Making sure the audible test is legitimate has its own set of challenges, but once set up, it can run considerably faster than full CISPR 25 sweeps.
LISN Explainer
LISNs are used in multiple different test methods, with different names and characteristics. What the heck are they, anyway?
I’m indebted to Ken Javor’s 2023 article “Line Impedance Stabilization is in its Seventieth Year and Still Going Strong”.
When we test equipment/boxes/modules for EMC, we are testing them in very different conditions than their installations. For instance, usually there’s only one module being powered by only one power supply. In a real installation, there would be power distribution points that feed many different modules (e.g., in a car, your average 12V module gets power from the body control module (BCM) instead of directly from the alternator or 12V battery, and the BCM may be sending power to dozens of modules). There’s a lot that will vary from installation to installation, depending on the platform, end use, construction, etc.
Enter the Line Impedance Stabilization Network (LISN), also sometimes known as an Artificial Network (AN). The clearest picture I’ve yet found to represent its purpose is the one below from GSFC-STD-7000B. The LISN is meant to represent Zs from the picture below.
If you assume that power is distributed via a single wire running 5 cm above structure, and structure is used for current return, then you can reasonably estimate 1 uH/m inductance from all that wiring. On a very large platform like a naval vessel, 50 m of wiring isn’t unheard of--and now you know the origin of the 50 uH LISN. The very first LISN design, from 1953, is the 5 uH LISN (aircraft in particular were smaller back then), and the 5 uH LISN is still used when appropriate today.
There are plenty of variations. For instance, the typical Goddard Space Flight Center project (JWST notwithstanding) is a small science satellite, a cube not much more than 2 m on a side, with a 28 Vdc battery recharged by solar arrays and never using structure for current return. That’s a very low inductance arrangement, and thus they use a stabilization network with a pair of 10 uF feedthrough caps and a 10,000 uF line-to-line cap.
LISNs also help in providing repeatability of measurements across tests and across different labs. Thus if you’re doing FCC testing, you’ll be using the LISN specified in ANSI C63.4 no matter what your final installation is.
TIP:
Take a moment to consider what flavor of LISN is most appropriate for the end installation your product will be used in. MIL-STD-461 Rev G Section A4.3.6 has tailoring guidance on this topic.
ANSI C63.4: “American National Standard for Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 9 kHz to 40 GHz”
ANSI C63.4 contains the main test methods used to verify compliance to FCC Part 15 unintentional emissions limits, making it key for consumer electronics.
ANSI is the American National Standards Institute, and C63 is its committee specializing in EMC matters, particularly test and measurement. There is close coordination between ANSI C63 and the IEEE EMC Society and IEEE Standards Association. C63.4 relates to measuring EM emissions from normal electrical and electronic equipment, and it can be purchased here. Most importantly, it is the standard referenced by the FCC (47CFR15) and Canada (ICES-003) for consumer electronics compliance to FCC Part 15 emissions limits. The current version is from 2014, and it is currently being revised for the next release. Which is a great time for us to remind you that it is free to participate in ANSI C63 standard working groups, so please get in touch with us if this is something you’d like to be involved with--we can put you in contact with the right people to inquire.
C63.4 is one of the broad standards, something that works towards being comprehensive instead of very narrow. For one thing, it deals with both conducted as well as radiated emissions. It covers the test equipment needed for this testing, which includes equipment for different test methods covering the same frequency range. It has a section on measuring IT equipment specifically before moving onto more generic methods. It also has a number of normative and informative Annexes providing more information, context, references, etc. The intent is always to protect nearby equipment, be that a Ham radio, a computer or a pacemaker, from interference.
The standard covers test instrumentation (including some very useful information about test antennas, the uses & limitations of different types), test site requirements, considerations for test setups, discussion of relaxations for sporadic transient emissions, and the following specific test methods:
AC power line conducted emissions, 150 kHz - 30 MHz
Radiated emissions, 9 kHz - 40 GHz (for FCC testing, only 30 MHz - 1 GHz is applicable)
Radio noise power using an absorbing clamp
Test setups specific to IT equipment
Recommendations for testing unintentional emitters other than IT equipment
Radiated emissions testing using TEM waveguides, 30 MHz - 1 GHz
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It should be noted that the testing done in ANSI C63.4 is significantly different than that found in the aerospace/defense/automotive sections. So a unit that qualifies to FCC limits can’t be automatically said to meet MIL-STD-461 RE102 or CE102, CISPR 25, or RTCA DO160 Section 21--or vice versa. For an explanation of the details, see this presentation Ms. Burnham gave to the SE Michigan chapter of the IEEE EMC Society early in 2024.
MIL-STD-461: “Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment”
MIL-STD-461 is one of the core EMC standards, having evolved with military applications since WWII. It is widely used in the aerospace and defense sectors.
MIL-STD-461 is a landmark document in the world of EMC standards, and there are a lot of other standards that derive from this one. The easiest place to find a copy of MIL-STD-461 (free, as most government standards are) is here, or the official site is here. The current edition is Rev G, and the working group for the standard is currently drafting Rev H.
MIL-STD-461 is more of a document that specifies test methods than a strict requirements document, and while it has suggested limits for many of the tests, in most cases those limits should be tailored. For general aerospace and defense projects MIL-STD-464 is the actual requirements document, and the tests in MIL-STD-461 are how you document compliance to the EMC requirements.
TIP:
Tailor, tailor, tailor! Beware of any project that simply tells you to “meet 461”--even accepting 461 as the overarching requirement, tailoring 461 is key for saving testing time and budget and not wasting resources designing to inappropriate or inapplicable requirements. I’ll discuss this more in articles dedicated to each test method.
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Read the appendices! More than most standards, the committee behind MIL-STD-461 documents background information for every section of the main document. This includes context, lessons learned, and why different changes have been made over time. There’s a wealth of information in there.
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MIL-STD-461 does not have a requirement to perform testing in a certified lab. While many of the labs that have the equipment needed to perform 461 testing are certified (to ISO, by ANLAB or A2LA, etc), that is not required. Testing in an uncertified lab or your own facility is allowed as long as you can meet the test equipment and test reporting requirements.