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I am designing a device that has no intention to transmit or receive any radio waves but it has CPU, large PWM driven LED indicators so may be some higher frequencies inside. To make sure we pass the EMC easily and without "iterations", I would like to test it (at least approximately) by our own engineers. We would like to know how exactly the measuring device is named so we could purchase it and what are the minimal specs for it. I tried to google that, there is a big variety of them and some are horribly expensive. Do I need exactly spectrum analyzer when analyzing the spectrum may not be my requirement?

What is the minimal required equipment for this task and how this could be done?

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    \$\begingroup\$ A spectrum analyser and a near field probe is the least you need equipment wise. You will need to do some wider field tests, with a spectrum analyser and a broadband antenna. You also need to understand what you're looking at, the fact you're asking this question suggest that will be your weak point. \$\endgroup\$
    – Puffafish
    Commented Sep 29, 2020 at 7:44
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    \$\begingroup\$ I'd like to say that if you have to ask what equipment you need (indeed a Spectrum Analyzer is the minimum) and that such equipment is expensive (it can be) then the better option for you and your colleagues would be to educate yourself on the subject of EMC and/or hire a 3rd party that is specialized in this field. ...that measure the voltage. Well, EMC is not only about "voltages" so you really need to educate yourself or just hire a company to help you with this. You might already know that having an EMC test done isn't cheap either. \$\endgroup\$ Commented Sep 29, 2020 at 7:52
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    \$\begingroup\$ Start educating yourself by watching these EEVBlog videos: youtube.com/… As opposed to many other Youtube channels, Dave from the EEVBlog knows what he's talking about. EMC isn't easy, you also need to understand EM fields etc. So, in my opinion, you can't (should not) just "buy the equipment" and do the test yourself, you need to understand what you're doing as well. \$\endgroup\$ Commented Sep 29, 2020 at 7:55
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    \$\begingroup\$ I think the most economical route is book a day of private testing at a lab and take note of what is happening during the tests. The lab guys I've met in the past have always been helpful at explaining things and you'll start to get a better understanding. Buying first is not going to work. \$\endgroup\$
    – Andy aka
    Commented Sep 29, 2020 at 8:06
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    \$\begingroup\$ Unlike other folks, I prefer written materials over videos. EMC background. \$\endgroup\$
    – JRE
    Commented Sep 29, 2020 at 8:13

2 Answers 2

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In short, almost any electronic equipment needs to pass conducted and radiated EMC compliance. A few items like a flashlight would be exempt (provided it doesn't use a switching driver).

Without wishing to dismiss your efforts, if you have little or no experience in this area, you'll need to employ the services of a decent EMC Consultant. Aside from anything else they'll be able to let you know what standards apply to your equipment.

Do you also understand product safety regulations ?

EMC compliance is a seriously non-trivial task although as you come to understand the important issues, it need not be that complicated. Certainly buying EMC test equipment without an understanding of the testing methods needed is very 'about face'.

It's VERY important to engage someone NOW, before you make potentially costly design errors (it's easily done) that frustrate any effort to minimise EMC issues.

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  • \$\begingroup\$ A large part of EMC compliance is at the DESIGN STAGE, Trying to fix it 'after the event' can be time consuming and costly. \$\endgroup\$ Commented Oct 8, 2020 at 10:04
  • \$\begingroup\$ have no resources to shelve $5000 for a single visit to a laboratory that will likely just discover that the device is not compliant if I do not care and do not measure myself in advance. This is enough money to buy spectrum analyzer, near field probe and a good book explaining how to use the two. Some money may even remain for a well worn electric drill. \$\endgroup\$
    – h22
    Commented Oct 8, 2020 at 10:50
  • \$\begingroup\$ Which country are you in ? It's FAR, FAR better to spend a few $$$ in advance to get some advice about getting your design correct, rather than blow a huge sum in a compliance lab with zero return ! Do you have any of the standards required ? e.g. UL IEC FCC \$\endgroup\$ Commented Oct 8, 2020 at 10:54
  • \$\begingroup\$ I am in Europe (Switzlerland) so the requirements would be CE. \$\endgroup\$
    – h22
    Commented Oct 8, 2020 at 11:49
  • \$\begingroup\$ CE defines the compliance regulations. Do you have the relevant EN's or IEC standards to hand ? Not sure if Switzerland uses ENs as it's not in the EU. Do you have any experience with EMC testing ? Anything you make needs to meet safety standards too. How early are you in the design cycle ? EMC compatibility ideally needs to be by design. \$\endgroup\$ Commented Oct 8, 2020 at 12:22
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This is called precompliance testing. It is useful, and recommended, because while less accurate and precise, it can detect design mistakes earlier than otherwise.

First of all you need a book you can trust. A book like Electromagnetic Compatibility Engineering by Henry W. Ott specifies exact requirements for European, American and Canadian regulations, recommended procedures, advises on design and contains in depth chapter on precompliance testing. It is from the reputable author and has many positive reviews in Amazon. Of course you also need to read it but there is nothing there impossible to understand. All other suggestions in this answer are from this source.

Antennas are not recommended in this kind of testing: they are large, sensitive to nearby reflections and interact with metal objects. It is more reliable to measure some parameter that is proportional to the radiated emission. Instead, small probes just few inches size are used. The shielded room is also not required and can introduce huge errors if wrongly implemented. It is possible to verify the source of EMC inference (our device or something external) by simply powering the device under testing down.

Cables are usually the major source of emission so clamp on probe around the device cable (if it has any) should be tried first. Most cable radiation occurs below 250 MHz. All cables must be measured for inference, regardless of they function. This is called common mode radiation. These measurements require the HF current probe clamped on the cable. The devices mentioned in the book cost unfortunately $2000+ even if used. Here is an innovative device below $1000 from Tektronix, wonder what it could do.

Another kind of inference results from current flowing around loops in PCB. This is called "differential mode radiation" and needs another probe. The source mentions probes with the range up to 500 MHz as useful. "Professional" probe set may cost about $1000 but they are actually very simple devices and can be hand made from several pieces of high frequency cable (the book contains the drawing). Anyway they cannot give numeric results for saying how much is the device compliant, but can help to find the source of the RF noise and would tell if you have a lot of it.

The most expensive part of equipment is the spectrum analyzer. The book source calls $15000 analyzer "ok" but there are more bearably priced devices on Alibaba.com. Some of them may be good enough for precompliance testing.

You can also do the noise voltage measurement on various power circuits. This can be done with high frequency oscilloscope. They are helpful in pinpointing the source of emission.

A simple precompliance testing for the radiated immunity can be done with some power tool containing a brushed electric motor. The arcing that occurs on brushes emits very broad range of electromagnetic noise. Move it 1 m from the PCB across the board and observe the behavior of your device.

A simple device for ESD (electrostatic discharge) tests can be made from pjezoelectric fire lighter.

Finally, if you really want to measure real radio emissions, pack your antenna, spectrum analyzer, device, portable power source and go somewhere deep into the woods where mobile phones are unlikely because there is no coverage.

Most of the approaches to achieve the EMC are not any secret and can be found in the literature. The problem is that in some cases simply adding ferrite bead or decoupling capacitor in a wrong way may even worsen the problem. Even rather approximate measurements would show if the situation has improved or getting worse.

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  • \$\begingroup\$ The most expensive part is a shielded room :) I have a spec & near field probes myself, but they are pretty useless outside a lab, since they pick up so many disturbances. I've given up on home-brewed precompliance testing and hire time in a lab instead. It's usually not -that- expensive when you do informal testing. And you can hire that lab a whole lot of times before the cost gets near the price of a quality spec. \$\endgroup\$
    – Lundin
    Commented Oct 1, 2020 at 8:34
  • \$\begingroup\$ In the Book it is written "what you definitely do not want to do is to build a shielded room, place your product and antenna inside and measure. This maximizes the errors". The Book advises to use probes instead. \$\endgroup\$
    – h22
    Commented Oct 1, 2020 at 8:51
  • \$\begingroup\$ Okay so how does the Holy Book suggest that you deal with intentional radiators from all over the place then? Many of them do not transmit constantly. In particular smart phones and misc 2.4GHz equipment. And when you attempt to measure the actual DUT and get a sudden spike, you have no way to tell the source. It's not as easy as knowing the carrier frequency of all intentional radiators in your city - you might as well get intermodulation + harmonics creating an unholy mess across the whole radio spectrum. \$\endgroup\$
    – Lundin
    Commented Oct 1, 2020 at 9:01
  • \$\begingroup\$ The Book says "useful measurements must be made such that they are not affected by uncontrolled environment", so probes. It is a good book by notable expert, with many positive reviews on Amazon. It cannot describe measurements that do not work in principle. \$\endgroup\$
    – h22
    Commented Oct 1, 2020 at 9:14
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    \$\begingroup\$ The Book says "turn your device off. If the noise remains this is due external pickup." \$\endgroup\$
    – h22
    Commented Oct 1, 2020 at 9:18

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