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By James Heilman, MD - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=34056919

Ventilator Projects and EMC Testing (EN 60601-1-2:2014)

Summary

If you haven’t already, check out part 1 of this blog Part 1: Rapidly Manufactured Ventilator (RMV) Projects and EMC Regulations

We’re going to take a look at the EMC requirements for RMVs, consider some of the risks posed by EMC and propose some methods of mitigating them.

Probably the biggest EMC risks are Radiated RF Immunity and ESD due to their higher than normal test levels.

If you need any fast turnaround design support and testing services for your Rapidly Manufactured Ventilator project then get in touch.

Background

As noted in the MHRA RMVS specification on 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”

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.

Emissions

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.

Immunity

Overview

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

Immunity Risks

There are two big risks to the immunity performance: Radiated RF Immunity and ESD.

Radiated RF Immunity

Test Requirements

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:

Frequency (MHz)ModulationTest Level (V/m)
38518 Hz pulse, 50%27
450FM +/- 5kHz dev.
1kHz sine
28
710, 745, 780217 Hz pulse, 50%9
810, 870, 930 18 Hz pulse, 50% 28
1720, 1845, 1970 217 Hz pulse, 50% 28
2450 217 Hz pulse, 50% 28
5240, 5500, 5785 217 Hz pulse, 50% 9

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:

  1. 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.
  2. 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 Recommendations

Design ideas to mitigate this interference include

  1. Keep traces/connection as short as possible between sensors and ADC
  2. If you can mount them all on the same circuit board then do so
  3. This circuit board will have one layer dedicated to a solid ground plane fill over the entire plane. All ground pins

    Check out my video presentation on PCB grounding and HF current flow.
  4. Cables = antennas that are good at receiving the interference. Minimise use of cables where possible.
  5. 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.
  6. Decouple the supply lines to the pressure sensor well
  7. 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.
  8. 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.

Risk Analysis

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.

radiated rf immunity susceptibility characteristics

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.

ESD

Overview

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.

Design Recommendations

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

Emissions

  • Radiated Emissions, Class A, 30MHz to 1GHz (EN 55011)
  • Mains Conducted Emissions, Class A, 150kHz to 30MHz (EN 55011)

Immunity

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

Conclusions

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.

Networking Equipment – EMC Radiated Emissions Problem Solving

I had an email from a customer that I’m working on some design consultancy work with, saying that one of their prototype products was having some radiated emissions problems at an accredited lab. Could I take a look?

Absolutely, EMC radiated emissions problem solving is my favourite part of the job! Ironically, it is usually the customers least favourite part!

Thankfully I had a slot free the next week so they bundled their kit into the car for the long drive “Up North” from their base in the South West of the UK.

After some tinkering, the equipment was set up in the chamber for some radiated emissions work. The first scan confirmed the problem levels and frequencies that had been observed at the other laboratory.

The problem areas from their last scan were at 35MHz, 80-90MHz and a broad band between 150MHz and 220MHz.

 

System Overview

The system was housed inside a nice aluminium case that was being used for CPU heatsinking and environmental protection as well as EMC shielding. A rough diagram of the internals shows a main PCB with a large CPU / memory block in the centre and a variety of cables leaving the PCB and the casing.

The main power cable housing also had two debug connections inside the same housing that weren’t being used in the field but were available for updating software and such like.

rwns overview diagram

As is so often the case, this product was in it’s final stages of the development life cycle, meaning that no major design changes were possible. These EMC problems would have to be resolved using easy to fit additional components. Thankfully I have plenty of things in stock to try out.

 

Emissions Analysis

There are two important characteristics about these emissions that show us where to look

  1. They are predominantly broadband, an indication of analogue noise e.g. DC/DC converter / power supply. Sometimes this broadband noise is generated by digital switching but this can be less common.
  2. They are all low in frequency, where large or long structures are the most efficient antennae. This usually means cables.

So power noise and cables…. hmmm…. any good ideas?

 

OK Kids, Let’s Take a Look at the Cables.

In a very sensible move by the designer, both the DC power and Ethernet cables had some common mode filtering on the PCB.

Ethernet magnetics have common mode chokes built into the transformer stack which reduces the noise emitted and increases the susceptibility performance of Ethernet despite the often unshielded twisted pair cables used.

The caveat is that once the cables have left the magnetics that they must be protected from other interference sources. Noise coupling on to these lines is going to be heading straight out of the enclosure using these lines as the antenna. Similarly, if common mode noise gets onto the centre-tap of the output side of the magnetics then this can also cause similar issues.

I have experienced system noise coupling on internally routed Ethernet cables before and it nearly always results in lots of low frequency emissions.

The power cable had a small surface mount Murata filter in place with excellent attenuation at the frequencies of interest.

murata filter characteristics and equivalent circuit

Both the Ethernet and power cables pass through the shielded enclosure with no connection or filtering to the case. In bypassing the quite nice Faraday cage of the enclosure, any noise current on these lines will inevitably appear as radiated emissions and be picked up by the receive antenna..

Now to find out some more info.

 

Radiated Emissions Experiments

First, unplugging the Ethernet cable dropped the emissions significantly from 30MHz to 120MHz.

Secondly, some messing around with ferrite cores on the power cable reduced the 150MHz to 220MHz hump down to sensible levels.

This left a single peak at 270MHz that was traced to noise using the coaxial RF cables to the antenna to radiate.

Lets look at each of the points in a bit more detail:

 

Ethernet

The only practical method of dealing with the Ethernet emissions was to change the bulkhead connector to a metallic screened version and the external cable to a SSTP (Screened Shielded Twisted Pair) type of cable. No exciting analysis here I’m afraid.

 

Details of the Power Cable Noise Coupling

The most interesting coupling mechanism was happening inside the un-screened bulkhead power connector. Thanks to the power filter on the PCB, there was very little noise being conducted back down the cable from this line. However, the debug connections to the CPU are picking up all kinds of noise and carrying that noise to the connector.

rwns cable coupling close up

Disconnecting and bundling the debug cables near the connector cuts the radiated emissions down to next to nothing.

What’s most interesting is that the capacitive coupling region between the power cable and the internal debug cables is so small. The connector is only 20mm long and the cables run parallel with each other for barely any distance. And yet there is enough noise current being coupled onto these lines that it causes a radiated emissions problem.

 

Details of the RF Antennae Noise Coupling

By the time that all of the cables had been filtered or removed, there remained just one emission at 270MHz that was failing the Class B limit. An investigation with RF current probes showed a lack of noise on the main output cables listed above, even when they were screened or filtered appropriately.

A wander round the enclosure with an electric near field probe and spectrum analyser showed a spike in emissions near the RF antenna housing on the side of the EUT.

 

rwns antenna spurious

Checking the antenna feed cables showed them connected to the PCB pretty centrally. Disconnecting the coaxial cables from their mating halves dropped the emissions down to the noise floor.

Even though the noise isn’t in-band for the antennae themselves, they still perform well enough to radiate the noise and cause an emissions problem.

 

Summary of Fixes Applied

The below diagram shows the fixes applied to the EUT to achieve a Class B pass.

rwns applied modifications

Firstly, a fully screened metal bulkhead Ethernet connector was chosen for use with a shielded cable. This isn’t ideal from the installation point of view but is ultimately unavoidable without more significant modifications to the EUT.

Secondly, a Wurth ferrite was equipped around all three of the cables connected to the power bulkhead connector. As detailed above, it is necessary to put the ferrite around all three cables and not just the power to reduce the noise entering the capacitive coupling region around the connector.

Thirdly, a small ferrite was placed around each of the UFL cables at the point at which the antenna cables left the housing. This is a fairly common modification for radiated emissions, one I’ve employed several times before, and there are numerous suppliers of ferrites of various lengths with just the right inside diameter for the type of thin coaxial cable used with UFL connectors.

 

Results

Closing Thoughts

Any time your cable passes through a shielded enclosure with no RF termination at that point, you can pretty much guarantee its going to need some filtering.

Nothing particularly in depth in this analysis of the EUT, but I did find the coupling in and around the power connector particularly interesting.

At the end of the day, the best outcome was a happier customer with a path forward for their product.

 

ram cage being removed from a 2018 mac mini

Apple Multi-Purpose EMC/EMI Shielding

I’ve always been impressed with Apple’s approach to reducing problems caused by EMC/EMI. Making top of the line technology in a compact case means minimising risk and maximising performance.

Let’s look at an example of well considered EMC design and why it is so useful.

 

Even the EMI shielding solutions are stylish

Because their products are charged at top dollar prices, they can afford to (or can’t afford NOT to) put in features like this.

The RAM on the new Mac Mini (thanks to iFixit for the great photos) has its own removable cage, secured to a PCB level counterpart with screws and, no doubt, a decent fit along the edges. What’s interesting is that this shielding system will have multiple functions.

Let’s discuss these below.

ram cage being removed from a 2018 mac mini

Image from iFixit

 

Why is the screening can so important?

Primarily, it will be used to reduce the EMC radiated emissions from the product. The Apple products I’ve had in my anechoic chamber have all been very quiet and this is why I hold Apple in some regard for their EMC design.

Apple will no doubt have tested their design with multiple RAM vendors to satisfy themselves that the design meets the requirements of international EMC standards.

However, were the user to install some non-Apple verified memory modules then the risk of emissions could increase. One can well imagine that Apple will have considered this in their EMC Risk Assessment.

The secondary benefit is more subtle. Take a look at this image.

inside shot of mac mini case with component analysis

Original image courtesy of iFixit, markup by author

The memory modules and their screening can are highlighted in red. Next to it, highlighted in green, is a smaller board level shielding and a UFL antenna connector. (There are another two connectors out of sight underneath the case)

That’s right, Apple have put the most noisy part of the system (RAM) right next to one of the most noise-sensitive (Wi-Fi). What?

 

Noisy Neighbours.

This is not an uncommon problem, especially when trying to compress so much functionality into such a small space.

The Mac Mini is only 165mm square (that’s 6.5″ if you are watching in black and white). The case includes an integrated mains power supply making proximity between electromagnetically incompatible systems unavoidable.

Modern RAM speeds are fast and the Mac mini is no exception. Everymac lists the latest Core i7 model with a DDR4 memory speed of 2.66GHz. That’s uncomfortably close to the Wi-Fi operating band of 2.4 to 2.5GHz.

The interference spectra of a DRAM interface fundamental frequency is generally quite wide band.

If you turn on any form of Spread Spectrum Clocking (SSC) to reduce the peak energy then it can spread over tens or hundreds of MHz. Either way, that puts the edges of the memory fundamental in band for the 802.11 a/b/g/n/ac interface on the Mac mini.

The harmonic emissions of the memory are also prevalent and it’s easy for these to fall in-band of a wireless interface like Wi-Fi. For instance the second harmonic of 2.66GHz is at 5.32GHz in the channel 64/68 region for 5GHz Wi-Fi. Big problems.

 

Improve Performance? The Can Can.

The effect of in band interference on a Wi-Fi interface can be subtle.

At it’s most gentle, there’s a reduction in both performance and range. The modulation, coding type and channel width of the Wi-Fi sets the robustness of the interface to interference.

At the other end of the scale, whole channels can be blocked out entirely.

This intra-system, or platform level interference is pernicious and can be difficult to isolate and track down. Low noise floor spectrum real-time analysers are extremely useful tools here.

Ultimately, segregating the noise source from the receiver, is the only real solution. This can be achieved by physically separating the aggressor and victim (not possible here) or by shielding.

For some companies, the fallout in performance of a couple of Wi-Fi channels is no big deal.

If you are Apple however, then you can’t afford to have dissatisfied customers complaining about poor Wi-Fi speeds. As always, the EMC budget has to be congruent with the product budget and the desired performance.

 

The Last Line Of Defence

Check out the textured surface between the mounting holes for the lid (blue highlight on the above photo). That will be an EMI seal to ensure good contact between lid and case. Not only a nice touch but an important one.

The Wi-Fi antenna is mounted on the outside of the shield so this circular lid actually screens the antenna further from the noisy internal circuitry of the mini.

Well done Apple. I’d love to see your Wi-Fi range testing results… please?

 

 

Simple RF Current Transformer for EMC / EMI Investigation

This post contains some background info related to the video I posted on YouTube on how to make a simple RF current transformer, a great tool for debugging EMC / EMI issues such as radiated emissions from cables, or tracing conducted RF immunity noise paths.

RF current transformers (or probes) are commercially available products from places like Fischer CC or Solar Electronics and they work really well, have specified bandwidth and power handling characteristics, built in shielding, robust case, etc.

They also cost a few hundred £$€ each which, if you are on a budget like most people, represents a significant investment for a individual or small laboratory. However, this one can be built very cheaply; most labs will have a development kit with some clip on ferrite cores, if not the core I used only costs £5 from RS.

DIY Current Probe

I’m a big fan of making my own test adaptors and equipment as its a great way to really understand how things work and the compromises in any design. As such I decided to share how I go about making this kind of really useful tool.

It’s primary use is for A-B comparison work; measuring the current, performing a modification and then measuring the current to see the improvement.

It is to be stressed that my version is a crude but effective piece of equipment and does not replace a well designed commercial product. There’s a time and a place to invest in quality equipment and one should use engineering judgement on when that is. For instance, measuring the RF current accurately is definitely a job for a properly designed and characterised device.

If you want to explore RF current transformers in more detail then there is plenty of info on Google, but these links are useful places to start.

Some of the design compromises involved in this low cost approach include:

Core Losses / Insertion Loss

The ferrite material in these cores is specifically designed to be lossy at the frequencies of interest, which will result in a lower reading than a higher bandwidth core and a reduction in the amount of noise on the cable downstream from the noise source. This can in some cases mask the effect you are trying to measure. The commercially available products use low loss, high bandwidth ferrite cores.

A high insertion loss also makes these parts more unsuitable for injecting noise into circuits for immunity testing. they can be calibrated for this task using a simple test setup (to be covered later)

Secondary Turns

Number of secondary turns controls sensitivity but the more you add, the inter-winding capacitance increases, decreasing the bandwidth of the tool. I generally use 5 or 6 turns to start with but I do have a 20 turn part made with micro coax on a solid core which also helps to deal with…

Capacitive pickup

From the cable under test to the secondary winding. Normally a split shield (so that it doesn’t appear as a shorted turn) is built in to commercial products. Guess what, that’s easy to do on this with a spot of copper tape or foil.

Not as Robust

Although a well designed product, the plastic hinges and clips on the cores are not designed for repeated opening and closing. The Wurth Elektronik system of a special key to open and close the core is much more robust at the expense of having to keep a few keys to hand for when they inevitably go missing. However these parts are so cheap and quick to make that a broken clip on core is no real obstacle.

Future Videos

I’ll be following this video with some hints and tips on how to use these devices effectively for finding radiated emissions problems and for looking at conducted RF immunity issues. Stay tuned.

Video and Construction Errata

The sharp eyed of you will have spotted that I originally assembled the BNC connector on the core so that it covered the key-way to open the clamp. I rectified this but didn’t film the change.

Also, you can wrap the wire round the core without removing it from the housing but that means you don’t have a nice flat surface to affix the BNC connector to. It does make it easier to close the clamp however so make your choice.

radiated emissions plot

RS-232 to USB Converters – EMC Problems Part Two

A while ago, I wrote about EMC immunity problems with USB to serial converters and how it was easy to fix with a small 100pF capacitor to ground on the TXD and RXD lines for a bit of filtering. Well, now I’ve found the opposite problem of EMC radiated emissions failures caused by these periodically problematic products.

In this case it appears to be harmonics of the 48MHz internal clock of a SiLabs CP2102 being conducted out of the converter on the TXD and RXD pins.

These little boards are generally used as development tools in a laboratory setting but there’s nothing to stop this IC or module being integrated into a product where these problems would manifest themselves.

The below plot shows the radiated emissionsbefore (light blue) and after (red). This module was connected to it’s host by 10cm unshielded wires, not an unreasonable application by any means.

radiated emissions plot

And what was the fix? Yep, you guessed it, some 0603 100pF capacitors on the output pins to ground. I bet that would help with immunity too! 😉

IoT EMC Radiated Emissions Investigations

A customer requested some support with one of their products, an IoT bridge device that takes various sensors and provides telemetry back to a central server using a GSM module. Some of the radio pre-compliance spurious emissions testing had suggested there might be some issues at certain frequencies.

After a couple of hours of radiated emissions measurements in the anechoic chamber and some bench work with some near field probes, I’d developed a pretty good idea of what was going on in terms of where the emissions were coming from and what their radiating mechanisms were.

Interestingly, there was a common theme to all of these emissions…

These features are common to a wide range of similar devices so some notes and a simple drawing (oddly I find sketching like this a good way to relax!) are presented in the hope it will give you some ideas about where your radiated emissions might be coming from.

The sketch shows a keypad board, a CPU board and a battery pack. Some other information is missing to permit a simpler drawing. All of these boards below sandwich together nicely into a plastic case which was the starting point for the investigation.

The problem frequencies identified were a 300MHz narrowband spike and a 250MHz broadband hump. Usually when I see broadband I think “power supply noise” and narrowband I think “digital noise”.

IoT module - emc radiated emissions analysis

Let’s take a wander around the device.

Capacitive plate near field probing around (A) showed higher than background levels of 300MHz noise around the front panel button board. Since this was a “dumb” board, the noise was probably coming from the main CPU board. The noise emanating from the cable (B) was not appreciably higher but when approaching the CPU/memory the noise increased, the clock line between the memory device and CPU being the highest.

Two possibilities were that there was crosstalk on the PCB at (C) or perhaps inside the CPU itself but without getting into more complex analysis the exact cause is not known. Apart from the power lines, there was no extra HF filtering on the data lines, just a series resistor on the I/O lines of the CPU. The addition of a small capacitor (e.g. 47pF, either 0402 or an array) on each line to circuit ground forms an RC filter to roll off any unwanted HF emissions like this. I generally advocate making provision for such devices on the PCB but not fitting them unless required – better to provision for and not need than to require a PCB re-spin later in the development cycle.

Moving the near field probe around the bottom of the case where the battery lives (D) showed the broad 250MHz hump present on the battery. Unplugging the battery pack made the emissions drop by 10dBuV/m and measuring with a high bandwidth passive probe showed broadband noise present on the outputs of the battery charger (E) from the switching converter. Some low-ohm ferrite beads in series with the battery terminals will help keep this noise on board and prevent common mode emissions from the battery and cables (F).

Lastly, the antenna was unplugged and some other broadband noise was found on the cable (G) at 360MHz, this time from the main 5V DC/DC converter on the main PCB.

 

Conclusion

So what is the common theme? All the radiation problems stem from cables connected to the main PCB. As soon as you add a cable to a system you are creating a conductor with a poorly controlled return path or “antenna” as they are sometimes known in the EMC department!

Treat any cable or connector leaving your PCB as an EMC hazard. You have less control over the HF return paths in the cable environment than you do on the PCB. Apply appropriate HF filtering to the lines on the cable and remember that even a shielded cable can cause problems.

Sometimes, like the antenna cable, there’s not a lot you can do about it other than practice good design partitioning to keep noisy sources away from the cable and to apply a ferrite core around the cable if it becomes a problem during testing.

 

I hope you found this useful and that it has given you some pointers for looking at your own designs with a new perspective.

 

Use of an LCD back panel as an image plane to reduce radiated emissions

EMC Radiated Emissions Fault Finding Case Study

I’m really happy to have one of my blog articles featured on Interference Technology.

Problem solving and fault finding EMC problems, especially radiated emissions, is one of my specialities and oddly enough is one of the facets of my job that I enjoy the most. After a successful exercise in helping a customer out with their product, getting the chance to write about it and share it with you is a real bonus.

Fixing radiated emissions is at it’s most challenging when the scope for modification to the unit are limited by the fact there are significant stock of PCBs or components that would require scrapping and redesign. Finding a way to use the existing stock was key in this example as the customer had significant time and money invested into the project. Thankfully I was able to help them out.

Head on over to Interference Technology and have a read through – I even put pictures in! Hopefully it will give you an idea of how I work and the sort of EMC issues that I can help you solve.

Case Study: Poor PC Board Layout Causes Radiated Emissions

 

Unit 3 Compliance anechoic chamber

Recent Work: Medical Laboratory Equipment EMC Testing and more

It has been an good couple of weeks here at Unit 3 Compliance with some very interesting products to work on.

Firstly, I received confirmation from a previous customer that their product has passed the required EMC certification testing at their accredited laboratory with the modifications that we incorporated during our problem resolution work. This is excellent news for all concerned!

Then things started off with some radiated emissions EMC testing and fault finding on quite a complex and clever piece of medical laboratory equipment with multiple interconnected boards, display, motors and servos. A few problems were identified and feedback given to the customer about potential improvements.

Noisy DC motors were again the theme in some more radiated emissions testing, requiring additional suppression on the motor terminals, made all the more challenging by the tight mechanical constraints of the product. Differential mode suppression on the terminals using ferrite beads to reduce the brush noise is the most effective solution but without a well defined RF return path to the brushes any noise reduction will ultimately be a compromise. More testing is being performed with some small filter PCBs mounted right on the motor terminals.

Lastly, our screened room test facility is almost completed and is being used for some mains and DC port conducted emissions testing with buck converter switching noise providing a challenge.

“Problems worthy of attack prove their worth by fighting back!” 

Unit 3 Compliance anechoic chamber

Radiated emissions fault finding and pre-compliance in the Unit 3 Compliance fully anechoic chamber

 

radiated emissions plot showing improvements made to customers product

Brushed DC Motor – EMC Radiated Emissions Problems and Improvements

I’ve been working with a customer whose product had significant broadband radiated emissions throughout the spectrum, helping them with fault finding and improving their EMC performance. Partial information from their certifying test lab appeared to show a regular harmonic series suggesting noise from a digital clock of some kind. However, near field probing with a spectrum analyser on the bench disproved this theory and identified the brushed DC motor as the cause. It is believed that a combination of the certifying test lab only performing one frequency sweep, the long cycle time of the EUT and the random arcing noise from the DC motor brushes caused a series of regular peaks to appear distributed throughout the spectrum.

In reality, both spectrum analyser measurements and the radiated emissions plot shown below show a wide range of broadband noise once the graph had multiple sweeps through the spectrum to enable it to become fully populated.

Working closely with the customer, we selected an alternate motor from the same manufacturer part with suppression capacitors built in to decouple the brush noise at the source; always the most efficient way of solving an EMC problem is to knock it on the head where is is being generated. Putting the EUT in the anechoic chamber showed that this provided a significant improvement, especially at higher frequencies but there remained an hump at around 150MHz to 300MHz.

We followed up with testing various ferrite cores around the DC motor power leads to further reduce the noise being conducted down the lead, leading to the selection of a small ferrite core with two turns around the motor DC supply leads.

Below can be seen the scan data showing before any modifications (blue) and after changing the motor and adding the ferrite core (green). Testing also revealed what appeared to be a satisfied customer 😉

radiated emissions plot showing improvements made to customers product