“No Mr. Bond, I expect you to solve this radiated emissions problem!”
* laser noises intensify *
I was doing some radiated emissions problem solving on a smart LCD module and found an issue that is not new but I haven’t encountered for a while.
In this case, the solution required two ferrites. One on the I/O cable to the module and one on the flexible cable between controller and LCD screen.
Adding only a single ferrite in some cases INCREASED the emissions rather than reducing them, presumably an effect where the addition of the ferrite changes the resonant frequency of either one leg or the entire antenna to the troublesome frequency at 192MHz.
This reinforces the approach of:
Always add new fixes to existing fixes already implemented. Whilst it might be the fifth change that worked, it might not have worked without the previous four.
Once the last fix is in place and validated as working only then can you try and figure out what combination is actually required to solve the problem
The last step can get very busy, particularly if there are a large number of modifications applied. It might only be worthwhile if some are particularly expensive or difficult for the customer to implement in production. Different fixes for different budgets!
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2022/01/antenna-notes.png561420James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2022-01-12 14:45:402022-01-12 14:45:40One Ferrite Is Not Enough
They are thick, have lots of words, make me feel clever, and stop my bookshelf from floating away. They often have the one thing that you are looking for.
On the other hand, they have far too many (big!) words, too many equations with no context or explanation. I find it very difficult to sit, read and quickly gain an intuitive understanding.
I prefer to communicate with pictures. This is why my presentations are image heavy and text light. I’ve sat through far too many “PowerPoint Karaoke” sessions where the presenter reads the words on the slide.
Also I love the format of cartoons and graphic novels but you rarely see them outside of the fiction sphere. I’ve recently been thinking about what a combination of a graphic novel and a text book would look like.
With the recent acquisition of an e-ink tablet with drawing stylus to replace my 74 different notebooks and notepads I started sketching out some ideas for a guide to using near field probes. A subject that I’m often asked about and is complementary to our free Pocket Probe Set that we give away at shows and to customers.
One thing turned into another and once I started drawing I couldn’t stop. You can download the full eBook from the link at the top of this page or by clicking here.
I’d be interested to hear your feedback on the format and content of this mini eBook – please get in touch and let me know. If there is a positive response then more content may follow.
Thanks and all the best
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2022/01/Near-Field-Probing-p40-summary.png777582James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2022-01-03 16:00:082022-01-04 07:55:17Graphical Guide to EMC: Near Field Probing (free eBook)
These devices operate at 2.4GHz, a very crowded frequency band with Wi-Fi, Bluetooth and Zigbee all fighting for a narrow, congested slice of spectrum.
One of the common issues faced by users of this band is that of intra-system interference, sometimes referred to as “self” or “platform” interference. This is where components in the same system interfere with each other, primarily due to their proximity.
[Note: The counterpart to intra-system (within the system) in this context would be inter-system interference (between separate systems), which is what the conventional EMC test regime of radiated and conducted emissions and immunity seek to characterise.]
This common problem is something that our customer knows all too well from helping their clients integrate these Zigbee products into the end application.
So, during a recent visit to our lab for some testing on a related product, we spent some time investigating this noise on a typical setup.
The setup in the below image is common to many users with a Raspberry Pi Model B and lots of stuff plugged in to the USB ports. In this case, a Zigbee adaptor (black case) and an USB3.0 SSD in close proximity.
These parts, including the spectrum analyser, is part of the customers in-house electronics development laboratory.
The effects of USB3.0 on the 2.4GHz spectrum are well known. A good example is this 2012 paper from Intel which
For this demo, we used a near field capacitive probe and a 2.4GHz antenna to measure noise in the 2.4GHz to 2.5GHz band local to the Raspberry Pi.
This demonstrated the degradation of the noise floor with various levels of system activity including
Measurement of system noise floor
Presence of a USB3.0 SSD running a large file transfer using the dd Linux command
Activation of the Raspberry Pi internal Wi-Fi
The below image shows three traces under these different conditions.
The conclusions we can draw about the in-band noise are:
Noise from the SSD raises the noise floor by approximately 10-20dB (a factor of x10 to x100)
The Wi-Fi transmission from the Pi is 40dB above the local noise floor. This will mask any received Zigbee signals from a remote transmitter.
In-Band vs Out-of-Band Sensitivity
Well designed radio systems are generally very robust to out-of-band interference i.e. anything outside of the narrow radio band that it is tuned to. For instance, a Zigbee radio system set to channel 20 (2.450GHz) will reject anything below 2.445GHz and above 2.455GHz.
Intra System Interference Diagnosis
Advice on diagnosing these issues is mostly outside the scope of this short blog. Differences in systems, components and ambient noise levels makes it impractical to offer guidance for all situations. However, some generic problem solving pointers are presented below.
A systematic approach to isolating the problem is required.
One of the primary rules of problem solving is to change only one thing at once and observe the effects.
In EMC terms, it is possible to change several things at once without realising it. Cable position, the specific port that a device is plugged into, location of nearby equipment and cables, even how firmly a connector is tightened will all make small differences that stack up. (Don’t use anything other than a torque spanner on those SMA connectors though!)
Another key rule is if you think something has made a difference, reverse the change and see if the problem re-occurs. Unless you can achieve consistency then you might be changing something else unintentionally, or the problem is caused by something outside of what you are changing.
Correlating the problem against time can help. Does it happen when something else happens (other devices on, or off, or switching, certain configurations, times of day, etc.) This can give clues.
Lastly, we should be looking for a significant step change in improvement to identify the issue. Phrases like “I think it made a bit of a difference but I’m not sure” indicates that we are dancing around the issue and not getting to the heart of it.
Ultimately, for a detailed understanding, the spectrum analyser is a key tool in gaining a proper grasp of this issue.
The solutions to the problem are simple yet sometimes difficult – a technical balance needs to be struck.
Use of Ethernet rather than Wi-Fi on the Raspberry Pi.
It is not practicable to synchronise transmission from the Raspberry Pi Wi-Fi with that of the Zigbee stick. The simplest way of ensuring the Wi-Fi does not interrupt the Zigbee transmissions is to disable the Wi-Fi and provide network connectivity via Ethernet instead.
Depending on the installation this might not always be practicable but it certainly is more reliable.
Separation of components
Moving the antenna away from the noise source is usually the best way to achieve increased performance.
In this instance, placing the module at the end of a USB cable and away from other electronic items is a good start.
Another option that is not as ideal: a good quality SMA extension cable could be used to extend the antenna away from the problem area. This introduces loss into the RF channel, reducing signal quality. Measurements made in our lab on a cheap extension cable from RS show a power reduction of 6.5dB at 2.4GHz for a 5m cable. This equates to a ratio of around 0.25 meaning we are broadcasting and receiving a quarter of the power we were before.
Also, it is still possible for the noise to couple onto the nearby module even without the antenna attached meaning the problem does not get entirely resolved.
Better quality components
Sourcing a bunch of cheap-as-possible parts from Amazon or eBay is likely to bring problems.
Using devices from big name manufacturers and buying from reputable sources helps. But, even reputable components are designed to a price point and can still cause problems if the other points in this blog are not taken into account.
USB cables can be a big source of the problem. Unshielded back shells (the part between cable screen and connector body) compromise the shielding to the point where their performance at high frequencies is equivalent to an unshielded cable.
The only way to tell if a cable is good quality is to perform an autopsy on the ends and check on the cable shielding
Remember that Pawson’s Law of Cable Quality states that the EMC performance is inversely proportional to the physical appearance. Braided covers, shiny plating, metal connector bodies, transparent mouldings etc are all indications of money spent on the OUTSIDE of the cable. EMC quality comes from the INSIDE and is not visible.
I’ve been helping a customer out with some EMC pre-compliance testing of their new domestic product which included a range of 3rd party modules.
One of these modules was an “RCWL-0516” 3GHz radar for motion detection. These modules are widely available but technical information is mostly reverse engineered by enthusiasts and hobbyists. The best collection of information seems to exist on this GitHub page.
The customer was very keen to use these devices but making some measurements and looking into the regulatory side meant that it got a Big Fat No from me.
EMC Radiated Emissions
Radiated emissions in the 1-6GHz band were in excess of the Average limit line by over 17dB.
This is normally OK for a radio system, as exceeding these limits is often required to achieve the desired range and operation. However this only works if there is a counter-part radio standard to refer to…
Analysis of the Regulatory Status of this device
No CE / UKCA marking applied to these devices – should not be sold in the EU / UK
No CE / UKCA marking Declaration of Conformity supplied by manufacturer – should not be sold in the EU / UK
3.1GHz is not a Harmonised Frequency band. Instead, it is licensed, and operation is only permitted in some countries. The key to the table is at the bottom.
Even when taking this table into account, this band is only for UWB Location Tracking Systems.
Following the documents further down the chain, the ECC/REC/(11)09 mentioned above refers to two documents:
TR 102 495-5 for use of Ultra Wide Band for location tracking operating in 3.4 to 4.8GHz. This device is not UWB and not operating entirely in this band.
ECC REPORT 120 requirements for UWB Detect-and-Avoid for operation in this band. This device has not such capabilities.
The only way that this radar device can be considered legal to operate is if it meets the Class B (domestic) emissions limits in the 1-6GHz band.
Currently this is not the case. With this example product, emissions will need to be reduced by 17dB or more to comply.
The oscillator used relies on parasitic components between PCB elements. Tolerance of components, PCB manufacturing tolerance, values over temperature means that frequency stable operation is not practicable.
Other motion detector products exist – I’ve not linked to any as I don’t want to unfairly endorse anything I’ve not investigated further or tested myself.
Anyway, I hope this clears up some of the questions about this device.
I can’t recommend using these devices at all. If you are going to use one of these then keep an eye out for interference with other systems. Don’t even bother if you want to make something that you can sell at the end of the process.
Cheap 3rd party modules like this are usually cheap for a reason.
Thanks to Charlie Blackham for pointing me in the right direction with the radio standards.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2021/09/RCWL-0516-board-image-from-github.com-jdesbonnet.png320320James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2021-09-20 18:01:532021-09-21 09:55:11Compliance Assessment of a RWCL-0516 Doppler Radar Motion Detector
Here is an interesting problem I had working on piece of industrial equipment. The customer had some conducted emissions failures at another EMC lab and needed some help resolving them.
The lessons from fixing this problem was that the first thought is not always the correct one, and that sometimes, all you need is a bit of green-and-yellow earth wire!
A block diagram of the system is shown below with the major components shaded.
An industrial power supply feeds power to the controller (a custom PCB connected to a Raspberry Pi) and to the power measurement board (measures the power consumed by the load).
Conducted emissions on both the Ethernet port and the AC mains port on the power measurement board were both dominated by a low frequency hump around 700kHz.
Notice how the shape or profile of the emissions is almost identical. To my mind, this points towards a single component in the system causing the same noise to be seen everywhere.
The first thing I wanted to do was to simplify the test setup as much as possible. I replaced the industrial power supply (often designed for Class A emissions performance) with my trusty Thandar TS3022S adjustable linear bench supply.
The idea here was to eliminate the industrial power supply from my inquiries.
Wow, what a big difference!
So the conclusion here is that the industrial power supply DC output is very noisy, that this noise is propagating through the system, and manifesting as conducted emissions on the outputs via a variety of coupling paths.
Differential Mode Filtering
Because conducted emissions noise in this lower frequency range tends to be differential in nature (+ve relative to -ve), my first thought was to implement a differential mode filter on the output of the power supply.
I’ve got a little filter prototype board that I use in situations like this. This pi filter was made up from two Panasonic FC series 470uF, 25V on either side of a Wurth 33uH iron powder inductor.
Unfortunately it did nothing to the emissions!
Could it be Common Mode?
This sounds like a obvious question to ask in hindsight. Most EMC problems are common mode in nature, I’m just used to thinking about LF conducted emissions as a differential mode problem.
Let’s try a common mode mains filter on the output of the power supply to see if this is indeed the case.
That’s much better! It looks like the problem was common mode noise after all.
This Time It Was Actually A Good Idea…
Common mode noise in this instance is current on both the DC output lines together. But, as I point out in one of my talks, current flows in a loop and always returns to the source. So where is this common mode current returning to? What is it’s reference?
Our common mode emissions measurements are being made in relation to the metalwork of our screened room test setup which is connected to the AC mains Protective Earth (PE).
The AC mains line to each LISN contains a PE connection and, inside the LISN, this is connected directly to the floor of the chamber.
Logically then, connecting the DC negative to the PE on the power supply will provide a shorter path for this common mode noise from the power supply.
Will this have the desired effect on emissions?
Yes. Yes it does.
Ooooooh, bloomin’ common mode noise. Not just for the higher frequencies but lower ones too!
This was a fun half day project fixing this particular problem. Much nicer to be able to recommend a low cost cable assembly than £$€ 20 worth of filter block.
If you’ve got any EMC problems then give me a call, I’d be happy to help.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2021/03/1-lf-common-mode-noise-system-overview-.png4031046James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2021-03-09 16:51:522021-03-16 08:39:07Low Frequency, Common Mode, Conducted Emissions
It’s a little known fact that Brian Wilson from the Beach boys was an environmental test engineer at Big Corp before pop stardom called him away from his first career.
He quickly realised that the hit parade wasn’t ready for ballads about Bellcore or ditties about humidity and switched to songs about the West Coast, cars and surfing. What could have been eh?
What is true however, is that whatever product it is that we make, at some point we have to release it from our development lab and into the big bad world. This raises the question: how well is it going to survive?
Why Perform Vibration Tests Anyway?
As we know, there are known knowns, known unknowns, unknown knowns and unknown unknowns…. y’ know?
“But James,” you cry, “surely I could just send my product via DHL / UPS / least favourite courier service (delete as appropriate) and achieve the same effect as testing?!”.
Well that’s very true, but I’m sure it’s going to be quite difficult to calibrate Dave the delivery driver, Vinnie the Van, and Winston the warehouse man for a consistent acceleration profile.
Testing is a simulation of what can happen in the big bad world. Like all simulations, it is a sensible average of many different situations. I’m not sure how these standards were originally derived but I’m sure more thought went into them than just kicking a development sample around the car park.
(If not actual thought, then at least a lot of tea and biscuits at the committee meetings, and that’s good enough for me)
Testing it primarily about mitigating risks, both of known risks and unknown risks. Whilst you might not be able to envisage a situation where your product is subject to a 1g, 30Hz vibration, real life might have other plans.
Primarily we are trying to quantify our knowns and unknowns.
Common Questions, Regular Requests
A version of one of these questions crops up every month or so.
“I want to do vibration testing, is there a standard that I can use?”
“What level of vibration do I need?”
“Do I need to do a random vibration test or is a swept test OK?”
“How do I simulate something being driven around in the back of a van?”
Vibration standards are quite often customer driven with a defined procurement specification. How close those specifications are to real life conditions depends on how much research that organisation has done or if they just pulled the numbers out of a hat. Often there is no way to get at the data or decisions that led to those choices and one must take them at face value.
Sometimes serious research goes into these levels, with car manufacturers employing multichannel data recorders and taking the latest model out for a test drive whilst getting tangled in accelerometer cables (safely I’m sure). This ends up being proprietary data so good luck getting your greasy mitts on it unless you happen to be working for them.
For the rest of us without a big budget how do we even make a start? Just go an buy every single BSI standard on vibration testing? Oh wait, we said without a big budget…
In the absence of any customer specification to work to, a good place to start in determining required levels and profiles is with the ETSI Environmental Engineering series of standards.
It’s a Splendid Smorgasbord of Shaky Situations
(or a Buffet of Battering if you will…)
The root standard ETSI EN 300 019-1-0 v2.1.2 (most recent at the time of writing) gives an introduction to the “Environmental conditions and environmental tests for telecommunications equipment”. Since this could easily be used to describe most modern electronic equipment then it is widely applicable.
This incredibly useful series of standards are provided for download free of charge, a much more welcome approach to the usual way that standards are sold at high prices.
Not only that but they don’t just cover vibration. They cover the expected environment in terms of
Temperature and rate of change of temperature
Solar and Heat radiation
Moving air speed (wind)
Weather conditions (driving rain, icing, etc)
Biological conditions (mold, rats, animals)
Concentrations of chemicals
Vibration and shock levels
That’s a pretty comprehensive list! The root standard contains a useful table that enables easy selection of a sub part appropriate for your product.
Each Class has two standards that relate to it:
Part 1 (suffix -1-x) specifies the expected environmental conditions for the situations (storage, transportation, in use) and locations (underground, on a ship, etc)
Part 2 (suffix -2-x) specifies the recommended test levels and methods/standards for each class.
This table has quick links to each of the Part 1 and Part 2 standards for the various classes for reference:
Part 1 – Environment Definition
Part 2 – Test Specification and Methods
Root Standard with background and general definitions
“Yes Robin, but how do we apply them to our (bat) product?”
Let’s invent an imaginary product.
The Monitor-o-Matic 9000 is a battery powered environmental monitor for indoor and outdoor locations. It gets periodically transported around site in the back of a van (maybe not always in its nice Peli case, naughty naughty)
In this instance we could reasonably apply:
Storage – the equipment is often kept in the van when not in use which can get pretty hot and cold. So we’ll apply “Class 1.2 Weather protected, not temperature controlled storage locations”
Transportation – with a bumpy van, and the M-o-M 9000 not always stored in its box means we’ll pick the harshest tests of “Class 2.3 Public transportation”
Usage – Non-Weather Protected Locations could be characterised as locations “…where transmitted vibrations are experienced from machines or passing vehicles. Higher level shocks may be experienced e.g. from adjacent machines.” We’ll choose “Class 4.1E: Non-weather protected locations – extended”
So lets pull out only the vibration requirements from each of these (bear in mind they also call up temperature, humidity, chemical resistance and other factors)
Unsurprisingly (some might say shockingly), transportation forms the biggest risk of vibration and shock to the product. We’ve now got some test levels to work with.
Note that some of the standards don’s account for the product being dropped from height. The storage standard does have a static load test simulating things being stored on top of the product.
Similarly the transportation test has the same static load requirements but adds free fall, toppling and rolling into the mix.
Questions to Ask Yourself
We normally ask ourselves what the expected use conditions are for a product. As with anything like this, we must also ask “what are the foreseeable misuse conditions?“.
Confession: I’ve Been Seeing Other Standards
This is just a simple guide to using the ETSI standards to make sensible design decisions in the absence of any other good information. However, many standards codify vibration and shock requirements, making the decisions a little easier.
EN 54 for Fire Alarm components has some quite challenging requirements for alarm components. In one shock test I performed for a customer on a smoke alarm the unit pinged straight off it’s base at the lowest shock level, necessitating some further work on the mounting (it subsequently passed no problems).
Military, Automotive and Aerospace sectors will definitely have their own vibration requirements and will commonly be clearly specified as part of the procurement specification.
Anyway, I hope you find this useful. Get in touch if you need some further guidance or need to ask any questions.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2019/01/300-019-1-0-environmental-and-vibration-table.png12361262James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-11-03 22:28:482020-11-03 22:35:16How Do I Choose Vibration Tests For My Product?
Like most long articles, this started off as a short one. It all stemmed from a customer question:
“We had some issues using a LED driver that could not cope with load dump and volt spikes. Do you have any provisional tests that could determine the circuit reliability? It doesn’t have to be to [ISO 7637-2]”
The ISO 7637-2 standard defines automotive conducted transient test pulses on vehicle power lines (12V or 24V). It is called up by standards including:
UNECE Regulation 10.06 for E-marking
EN 50498 (aftermarket automotive equipment)
ISO 13766-1 (earth-moving and building construction machinery)
I don’t have an ISO 7637-2 pulse generator (edit: I do now!). Automotive surge generators are less commonly found in many EMC test labs due to their more specialised nature.
Systems are available to hire; budget for €/£1000/week for a generator that will cover Pulses 1, 2 and 3. They are also available to buy new; expect to pay around €/£15k. If you need to cover pulse 4 then this will increase the costs yet again, mostly for the bipolar amplifier.
But, like most EMC test labs, I do have an IEC 61000-4-4 (EFT) and IEC 61000-4-5 (Surge) generator capable of 1.2/50us and 10/700us pulses.
Question: Could I use the IEC generator to simulate the surge pulses from the ISO generator?
This question comes with caveats:
The aim here is pre-compliance / confidence testing with the tools available. Not to replace the ISO 7637-2 tests entirely.
We are only looking at the potentially destructive Pulses 1, 2a, 3a and 3b.
Unit 3 Compliance can perform pre-compliance and full CE Marking testing to EN 50498. We can also perform pre-compliance testing for many of the R10 tests for E marked products.
IEC 10/700 pulse generator can be used as a close substitution for a 12V system
For a 24V system the 10/700 pulse is not as good a match. Follow the flowchart to select the test compromise and set the surge voltage based on the values in the tables.
ISO Pulse 2a
Not a good match, recommend a compromise between current and energy as shown in these tables
ISO Pulse 3a, 3b
IEC EFT generator is a good match and can be substituted for ISO pulse 3a and 3b
Pulse Parameter Comparison
Comparing the pulse widths and impedances against each other gives a mixed picture.
For Pulse 1, neither waveform is a great match with both of the ISO pulses having a longer pulse width than the 10/700 generator. Whilst the 24V bus pulse has a much higher impedance, this could be corrected with an additional series resistor in the IEC generator output.
For Pulse 2a, the 1.2/50us IEC generator appears to be an excellent match.
For Pulse 3a and 3b, the 5/50ns EFT generator is pretty close but the width of the ISO pulse is three times bigger.
However, as we shall see below, this approach is incorrect as it does not tell the whole story.
Pulse Width Definition
The problem comes from how the pulse widths are defined in the standards. Let’s take the comparison between ISO Pulse 1 to IEC 10/700 comparison as an example.
e can see that the ISO pulse width is defined at the 10% crossing point, whereas the IEC pulse width is defined at the 50% crossing point.
This is not helpful.
How do we compare a ISO 1000us @ 10% with a IEC 700us @ 50% waveform?
Open Circuit Ideal Waveform Comparison
I found some information over on the PSCAD website that showed the equation for the waveshape (from IEC 61000-4-5)…
…along with some Matlab optimised coefficients for alpha, beta and k.
From the PSCAD website “Standard Surge Waveforms” https://www.pscad.com/webhelp/Master_Library_Models/CSMF/Surge_Generators/Wavelet_Transformation_(WT).htm
ISO 7637-2:2011 gives the equation for the falling edge only of the pulse waveform. It also states that “The influence of the rise time is not taken into account (tr << td), which is allowed for all pulses specified in this part of ISO 7637”
I also modified the equation for the IEC waveshape equation to take into account the generator and load impedances by taking the first term of the ISO equation and adding it to the start of the IEC equation.
A required surge voltage of 1V was used for simple direct comparison.
For Pulse 1 we can see that the 10/700 IEC waveform is actually a really good match for Pulse 1 for a 12V bus.
The same cannot be said for the 24V bus requirement. Some further thinking is required here.
The 55 ohm impedance for the 24V version of the pulse is the 15 ohm 10/700 generator natural impedance with a series 40R resistor in addition.
Despite Pulse 2 looking like a good comparison initially, the modelling shows that it is actually a very poor match.
For Pulse 3, the IEC EFT generator is a very good match and should be able to be used without any issue
Dealing With Pulse 1 (24V) and Pulse 2a
How could we go about compensating for the poor match between Pulse 1 (24V) and 10/700 IEC and between Pulse 2a and 1.2/50 IEC?
We need to ask ourselves: are we more interested in the peak voltage & current or pulse energy?
To answer this, first we need to understand the power input design of the Equipment Under Test (EUT)
EUT Design Assessment
It is useful to establish the following EUT design parameters:
Is there a discrete reverse protection diode? What is the Vrrm and Trr rating (reverse recovery time) of this part?
What is the maximum clamping voltage of the TVS diode and can the downstream circuitry survive this voltage?
It is important to remember that Pulse 1 is a negative going pulse caused by the disconnection of a large inductive load in parallel on the vehicle power bus. If the EUT has a reverse protection diode fitted then it’s Vrrm and Trr will change the effect of the test on the EUT.
It is also important to test at full current consumption if a reverse recovery diode is present as this will affect recovery time and therefore surge performance.
EUT Surge Suppression
The assumption is that we are testing an EUT that contains some basic low voltage electronics of some kind. The extension of this assumption is that it has some kind of surge suppression component connected across the power inputs.
This could be a Metal Oxide Varistor (MOV) or a Transient Voltage Suppression Device (TVS). These have a non-linear impedance with voltage and will restrict or “clamp” the input voltage to a defined level. Perhaps a component like a SMBJ26CA-TR.
This clamping voltage is dictated by the impedance of the part when conducting. This would be a diode-like VI curve for a TVS or the current-dependant resistor of a MOV.
Peak current is dictated by available peak voltage and generator impedance. So we need to be interested in the peak current to ensure that the correct clamping voltage is met.
Also, because the MOV or TVS absorbs some of the pulse energy internally, these components will have a datasheet rating for pulse energy. Exceeding this could cause significant damage to the part and affect its capability to handle future surges.
Pulse 1 Peak Voltage & Current or Pulse Energy?
Our main tools for adjusting an IEC pulse to suit an ISO pulse are:
The surge generator has an easily adjustable peak voltage through the control panel or software so this is the main method that will be used.
The Peak voltage is a significant consideration if the system has the reverse protection diode but the compromise test will depend on it’s voltage rating.
I’ve produced a flowchart to help selection of the right test level for using IEC 10/700 instead of ISO Pulse 1
Pulse 1 Best Compromise Voltage
I ran some more simulations in Geogebra adjusting the ratio between the IEC and ISO peak voltages and tabulated the results.
The best compromise is to minimise the total difference between current and voltage when expressed as ratios. This works out at a V_iec or around 0.6 * V_iso.
This yields the following test voltages, peak currents and pulse energies for the different severity levels.
It is interesting that the series impedance for the 24V version of ISO Pulse 1 is up at 50 ohms. This higher impedance implies that the surge expected in such a system would be induced from a parallel adjacent cable in a wiring loom rather than something directly connected to the ignition switch / inductive load circuit directly.
Pulse 2a Best Compromise Voltage
Same approach as for Pulse 1
Test Practicalities & Further Compromises
Pulse 1 Power Disconnection
The waveform for Pulse 1 shows a synchronised disconnection from the DC supply and application of the surge voltage. Since this is not easily done without
It is the surge pulse that will cause the damage rather than the momentary disconnection of voltage therefore, for these compromise tests, this is being ignored.
Coupling/Decoupling Network Requirements
The CDN inside the IEC test generator for mains coupling is adequate for the task of decoupling but the options inside my KeyTek ECAT test generator preclude the coupling of the 10/700 waveform. Instead, some creative front panel wiring with banana plugs will be required.
Since this CDN is designed for decoupling of surge and EFT impulses from the mains, I’m sure it will adequately protect the 12V linear power supply being used and also prevent the power connection from unduly affecting the test.
In may case, input is through a 16A IEC mains plug/socket but it is easy to make an adaptor. Output is via a BS1363 socket or, more convieniently, 4mm banana plugs.
This took way longer to research and write that I was hoping. Something in the order of three days of work was spent going backwards and forwards, thinking about it whilst doing DIY at home (nearly painting the cat as a result) and half listening to Tiger King on the TV.
I’m quite pleased with the result and I hope this eventually proves useful to someone.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/06/iso-7637-2-pulse-1-vs-iec-61000-4-5-waveform-comparison.png8861931James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-06-29 19:36:022020-10-19 08:10:37IEC Surge/EFT Generators for ISO 7637-2 Automotive Pre-Compliance
The LVD a.c. voltage limits are defined in terms of rms. From 2014/35/EU
This Directive shall apply to electrical equipment designed for use with a voltage rating of between 50 and 1 000 V for alternating current and between 75 and 1 500 V for direct current
So that’s 50Vrms and 1000Vrms.
A customer was asking about a low current (sub 1mA) 40Vac rms source and if the LVD applied.
This voltage is less than the 50Vrms threshold mentioned above so would be technically exempt from the LVD. Unless of course the equipment contained a radio module in which case the RED makes the LVD applies with no lower voltage limit.
I think that there is still a risk that needs to be assessed here.
Just because you are exempt from the directive doesn’t mean you are exempt from making your product as safe as possible.
Looking at the main table from 62368-1 for categorising shock risk, 40Vrms/50Hz means it is an ES2 hazard. The voltage source would have to be under this limit for normal operation, abnormal operation (e.g. blocked vent, stalled motor, controls set incorrectly) and single fault (open/short circuit) conditions.
This means, even though the LVD is not strictly not applicable, it would be wise to put in a Basic Safeguard between the user and the exposed voltage.
Additional: The provisions of the General Product Safety Directive (2001/95/EC) would apply to any product falling outside of any specific safety standard. The Harmonised Standards for this Directive include EN 60065 and EN 60950-1. Since both of these have been superseded by EN 62368-1 it would be reasonable to use this standard instead.
Thanks to Charlie Blackham from Sulis Consultants for the tip.
This safeguard could be an enclosure, insulation, an interlock or barrier.
Instructions or PPE aren’t sufficient as they are considered supplementary safeguards.
But what about the current limits?
That’s just considering the voltage source purely from a voltage perspective. If it can’t drive enough current into a 2000 ohm load for more than 2 seconds to form a hazard then that might change the classification.
This current is measured using one of the appropriate networks from EN 60990 such as the one below
But I know what I’m doing…
The requirement for safeguards depends on if you classify the user as a normal person or an instructed person
Skilled person > instructed person > normal person
220.127.116.11 instructed person person instructed or supervised by a skilled person as to energy sources and who can
responsibly use equipment safeguards and precautionary safeguards with respect to those
ordinary person person who is neither a skilled person nor an instructed person
skilled person person with relevant education or experience to enable him or her to identify hazards and to
take appropriate actions to reduce the risks of injury to themselves and others
The level of safeguard required between the user and the ES2 hazard is defined in EN 62368-1
For a normal person we must use a basic safeguard
But for an instructed person we may use a precautionary safeguard
A Precautionary safegard (defined in 0.5.5.3) could take the form of instructions or training, but the addition of warning stickers, PPE could also be considered part of this.
This is why I like EN 62368-1 over some of the older safety standards (I’m looking at you, 60950)
Rather than a prescriptive “thou shalt use 2.5mm clearance or be smote verily” it helps and guides you through all the steps into understanding why or how a requirement is derived.
Also the companion EN 62368-2 explanatory document contains even more background and context. I wouldn’t recommend applying -1 without having -2 to hand.
Stay SAFE kids.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/06/table4.png3781237James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-06-18 11:02:402020-06-26 08:49:23LVD Voltage Limits are RMS + Thoughts on Marginal Voltages
“EMC Testing (TBC): Must comply with IEC 60601-1-2:2014, Medical electrical equipment — Part 1-2: General requirements for basic safety and essential performance — Collateral Standard: Electromagnetic disturbances — Requirements and tests”
So lets take a look at a the EMC tests that might be required for a typical Rapidly Manufactured Ventilator project.
This is with the view of meeting the Essential Performance / Basic Safety requirements of EN 60601 whilst addressing the highest risk items first. This is prioritising speed of testing instead of performing a belt and braces, test everything approach that would be the common approach for Medical Devices.
These ventilators are going to be used in a hospital / clinical care environment under medical supervised use and not in a home environment.
60601-1-2 classifies a hospital as a Class A emissions environment for Radiated and Conducted emissions. This means that less time needs to be spent fighting to get the emissions below Class B.
Risk items to radiated emissions could include any brushed DC pump motors as these are notoriously noisy. Ferrite cores may be required around motor cables to mitigate this noise.
Following the design guidelines further down this article for any PCBs is recommended and will greatly assist with reducing EMC radiated emissions.
Most RMVs will be using an off the shelf power supply already approved to EN 60601 for Safety and EMC. AC Power conducted emissions should therefore look after itself and won’t be a significant worry for testing.
For Harmonic Distortion and Flicker, there is an interesting note in EN 60601-1-2 in Annex A
“It is assumed that ME EQUIPMENT and ME SYSTEMS used in hospitals (and large clinics) are not connected to the PUBLIC MAINS NETWORK.”
If this is the case, then Harmonics and Flicker requirements and tests need not apply as these only relate to the public mains network.
Most ventilator systems have no external electrical ports apart from the power supply. They are mostly self contained units. This greatly speeds up and simplifies the testing, and reduces the risk of problems with Signal Input / Signal Output Ports (SIP/SOP Ports in the standard, analogous to a Signal Port from other EMC standards).
Mains borne interference (EFT, Surge, Conducted RF) should be handled by the EN 60601 pre-approved power supply without issue. It will still need checking but ultimately the risk is low.
Dips and Interrupts and the hold up time of the power supply is something that would need considering at the Risk Analysis level to derive the correct immunity criteria for each of the individual tests.
If this needs improving then selecting a slightly larger power supply than nominally required could help. More likely, additional bulk decoupling on the main power rail (e.g. 1000uF) will help maintain the system DC voltage under these conditions.
Immunity Performance Criteria
Caveat: This section is me thinking aloud as I have no domain specific knowledge for Risk Management and Medical Devices. I’m trying to approach this from a common sense perspective to aid anyone working on a RMV.
The function of the EMC Immunity tests for Medical Devices is to ensure that the Essential Functions continue to operate and that Basic Safety condition is maintained.
Normally the immunity performance criteria would be based on the type of EM phenomena being simulated in the test. This is normally Criteria A for continuous phenomena (radiated or conducted RF immunity) or Criteria B for momentary phenomena (ESD, EFT, Surge, Dips/Interrupts). Criteria C only tends to crop up for longer duration power interruptions.
In the case of a Medical Device, maintaining the Essential Performance is the key parameter. If a momentary EM phenomena causes this to happen then this is a major problem.
Therefore immunity performance criteria must be considered for the key function of the device as well as the duration of the EM phenomena.
Based on this thought process, a sensible starting point for the immunity criteria is:
Assume criteria A (unaffected performance) for:
Key function of assisting patient breathing for all tests. This includes momentary EM phenomena tests = ESD, EFT, Surge, Dips/Interrupts.
Non-critical functions under continuous EM phenomena tests = Radiated and Conducted RF Immunity
Assume Criteria B for:
Key function performance for this means that there should be a function in the RVM firmware that remembers its last current operating state and settings and that it starts up in that state from a power cycle. This creates a requirement for programmable non-volatile memory (some kind of EEPROM) in the RVM.
All momentary EM phenomena tests for non-critical functions e.g. display readout may temporarily distort or flicker so long as it recovers
Assume Criteria C for:
Non-critical functions from momentary power loss e.g. screen/display readout or setting
There are two big risks to the immunity performance: Radiated RF Immunity and ESD.
Radiated RF Immunity
The basic requirement for radiated RF immunity is a flat 3V/m from 80MHz to 2.7GHz. So far so good, this is a fairly easy test to meet.
Now the bad news. Table 9 gives a list of spot frequencies to be tested to simulate close range exposure to common wireless technology standards. The table is summarised here:
Test Level (V/m)
18 Hz pulse, 50%
FM +/- 5kHz dev. 1kHz sine
710, 745, 780
217 Hz pulse, 50%
810, 870, 930
18 Hz pulse, 50%
1720, 1845, 1970
217 Hz pulse, 50%
217 Hz pulse, 50%
5240, 5500, 5785
217 Hz pulse, 50%
As you can see, this has testing up to 28V/m, a significantly higher field strength than 3V/m!
Risks to the EUT
This test loves to mess with analogue sensors. In the case of ventilators, the pressure sensors used frequently have an analogue output to a DAC on the CPU. This presents two risk areas:
Demodulation of noise inside the pressure sensor amplifier. This takes the small transducer signals and amplifies it up to the output voltage. Noise demodulated here would cause the carrier to be superimposed on the pressure readings.
The input of the ADC could be susceptible to noise picked up on the analogue voltage from the pressure sensor, even if the pressure sensor itself is unaffected. This will affect the readings.
Since the airflow and pressure sensors are a key component to the operation of the ventilator, these must be protected at all cost.
Design ideas to mitigate this interference include
Keep traces/connection as short as possible between sensors and ADC
If you can mount them all on the same circuit board then do so
This circuit board will have one layer dedicated to a solid ground plane fill over the entire plane. All ground pins
Cables = antennas that are good at receiving the interference. Minimise use of cables where possible.
Figure out what your minimum bandwith requirements for airflow are and filter the signal appropriately. You probably won’t need to sample the airflow faster than 10kHz so put a low pass filter right next to the ADC input. Something like a 4k7 and a 1nF will give you a 3dB of 34kHz. This will reduce the risk of RF noise being demodulated by the ADC input.
Decouple the supply lines to the pressure sensor well
Add a small filter to the pressure sensor input, perhaps another RC filter as shown above. This will help prevent the pressure sensor from being affected by the test.
It is possible that the pressure sensor will be directly affected by the radiated noise picked up by the sensor body itself and not by the traces. It would be prudent to provide a PCB footprint for a shielding can near the sensor. I have seen this effect on gas sensors in the past.
Assuming that the advice above is followed, the risk to the EUT is manageable.
One of the interesting features of Radiated RF Immunity testing is that of the Problem Band where most issues occur.
Most of the time, the problem band is in the 100MHz to 300MHz area (I’ll cover this in more detail in a future article). Cables tend to be the best antennae at these frequencies and, hopefully, our ventilator only has one cable of interest – the AC power cable. This has plenty of filtering for conducted emissions reduction which should handle this noise.
Probably the two biggest problem frequencies from the spot frequencies above are going to be 385 MHz and 450 MHz.
Then we are into the realms of direct pickup on internal signal cables and PCB traces at higher frequencies. If we’ve laid out our PCB well as highlighted above (short analogue traces, filtering, good ground plane, shielding provision) then this will help mitigate our risks.
The levels of ESD testing are almost twice that of the regular EMC standards with a requirement for 8kV contact and 15kV air discharges.
ESD is very good at upsetting digital systems and it has a particular fondness for edge triggered pins e.g. reset lines and interrupts.
If the reset line for the CPU controlling the RVM is shared with other digital circuit blocks or supervisory controllers then an RC low pass filter at the input to the CPU is highly recommended. This helps prevents unwanted resets.
Checking can be implemented in the Interrupt Service Routine to ensure that an interrupt condition actually exists, effectively de-bouncing the input.
Thankfully the Ingress Protection requirements for the RVM of IP22 and the requirement to provide flat, easily cleanable surfaces will probably dictate the use of some kind sealed membrane keyswitch panel. These have good ESD immunity as no direct contact discharge can take place on an switch where the plastic covering remains in place.
Whatever user interface technology the RVM employs, this will be a key risk area for ESD. If this is on a separate PCB to the main controller, all interfaces will need some kind of filtering. A small capacitor to ground on each of the lines that goes to the keypad would be a good idea. 0603, 100pF usually works well here.
Lastly on the mechanical design, keeping the electronics well away from the enclosure seams will also reduce the risks of creepage of any discharge into the circuit board.
Summary Test Plan
Radiated Emissions, Class A, 30MHz to 1GHz (EN 55011)
Mains Conducted Emissions, Class A, 150kHz to 30MHz (EN 55011)
Text in bold is highlighted as a risk item.
ESD, (EN 61000-4-2), 8kV contact, 2/4/8/15kV air. Test to connectors as well.
Radiated RF Immunity (EN 61000-4-3)
80MHz to 2.7GHz @ 3V/m
Various spot frequencies at up to 27V/m
EFT (EN 61000-4-4), AC Mains Port, 2kV
Surge (EN 61000-4-5), AC Mains Port, 1kV line-to-line, 2kV line-to-ground
Conducted RF Immunity (EN 61000-4-6), AC Mains Port, 3V/m (6V/m in ISM bands)
Dips and Interrupts (EN 61000-4-11), AC Mains Port, various
Not only has this article identified key EMC risks to Rapidly Manufactured Ventilators but also provided some design guidelines to dealing with the problems that might arise.
Some of the guidelines within might be useful to anyone designing a Medical Device. We haven’t covered the requirements for Patient Coupled Ports or SIP/SOP ports from an EMC perspective as they aren’t of too critical a concern for an RVM.
We can see how looking at the standard and pulling out the required tests can help us understand the risks involved in the design.
Experience of knowing how the tests will typically affect the EUT is the key to unlocking good design practices. In my case, this comes from having worked on many designs with problems and the learning that comes from fixing the issues that crop up.
Remember EMC test success comes from good EMC design. For a time critical RVM there is one chance to get it right – no do-overs!
I hope you found this article useful. See you when all this has calmed down. All the best, James.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/04/BIPAP.jpg7981024James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-04-18 15:55:522020-04-18 16:05:00Ventilator Projects and EMC Testing (EN 60601-1-2:2014)
1. CE marking probably won’t be necessary for UK for temporary ventilators 2. Still need to apply to MHRA for permission to use in clinical setting 3. This will likely involve the supply of test data to MHRA 4. The requirement for EMC testing is currently To Be Confirmed 5. U3C will give any ventilator design a pro bono EMC design review
In the current Covid-19 pandemic there are many project teams around the world coming up with designs for ventilators. The potential is for many such devices to be required.
The problem that this article identifies is that for normal medical devices, the length of time taken to get products approved is defined in terms of months. This is especially true for ventilators which are, according to several sources, a Class IIb device requiring a robust quality system and Notified Body approval.
The current requirement is for devices to be delivered within weeks.
The procedure covered under the “Exceptional use of non-CE marked medical devices” involves applying to the MHRA directly for a pass to supply this product for use without CE marking. This request will have to include data that shows compliance with the test criteria in Appendix B of the MHRA specification.
Page 11: “3. When the current emergency has passed these devices will NOT be usable for routine care unless they have been CE marked through the Medical Device Regulations. The device must display a prominent indelible label to this effect.“
Page 24: “EMC Testing (TBC): Must comply with IEC 60601-1-2:2014, Medical electrical equipment — Part 1-2: General requirements for basic safety and essential performance — Collateral Standard: Electromagnetic disturbances — Requirements and tests”
Note that the requirement for EMC testing remains TBC = To Be Confirmed. I strongly feel that at least some testing should take place, but functionality of the ventilator has to come first.
Next article will be a breakdown of the EMC test requirements called up in EN 60601-1-2:2014. I’ll also highlight some risk areas that designers of these ventilators will need to to bear in mind.
If anyone has any need for pro bono EMC design review services for ventilator projects or needs some EMC testing turning around quickly then get in touch.