This is a very useful analogy to use when considering an EMC emissions problem, particularly true for radiated emissions in the (often problematic) 30MHz to 1GHz band.
Lets get squeezing.
Many of you will have experienced this before. Making a change to an emitting structure inside the equipment by changing the electrical connection between two points results in some emissions going down and some going up.
Then you make another change and this has the opposite effect.
This is like squeezing our bag of water. We can move the water around in the bag much like we can emissions around in the spectrum. The harder we press down in one area, the more it pops up in another.
Emission goes up.
Emission goes down.
Reducing the volume
But unless we reduce the amount of water in the bag we will nearly always have a problem. The water is incompressible and it just finds new places to appear.
To achieve this in an EMC context we need to reduce the overall energy in the system.
This could be achieved either by keeping the energy controlled on a PCB away from the radiating structure or by adding lossy components (filters, ferrites, etc) to reduce the amount of energy coupling into the radiating structure.
Changing grounding and bonding within a system without reducing the energy is going to be an exercise in frustration and probably wasted time. Better to address the problem at source where possible.
There will always be a requirement for us to have to try and achieve the goal of “shaping” our bag of water to fit the radiated emissions limits.
A good example is a manufacturer that has already built a production run of units and needs a quick fix to get them onto the market.
Whilst this is often achievable, there are often significant rework / modification costs involved.
There is also the question of repeatability and consistency. If small changes in bonding of parts can make a large difference to emissions, how can you guarantee that each unit will be compliant? Testing multiple samples can help. As can having good production inspection points during the manufacturing process.
But common mode noise is a slippery customer and these kind of fixes should only ever be considered as temporary pending design changes to address the root cause of the issue.
A small plug.
Help is available.
We are really good at this kind of work
We’ve been through the cycle many many times with many many different products.
Using Unit 3 Compliance to help with your emissions problems gets you access to our years of accumulated experience.
Our on site test lab allows us to have a rapid cycle time between analysis of a problem on the bench, developing a fix, and testing in the chamber.
Hope this was interesting!
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2021/06/top-goes-down.png8101042James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2021-06-05 12:11:072021-06-05 21:00:13A Bag of Water.
I had a challenging EMC problem solving project in the lab this week.
A customer making a miniaturised 4 cm^3 buck-boost DC/DC converter for Li-Ion battery charging was having radiated emissions issues. The small size meant that adding common mode chokes to filter the input and output connections wasn’t practicable so a more in depth investigation was required.
How bad is it?
Here are the emissions for the EUT without any modifications. The green reference trace is the AC/DC mains power supply being used to power the EUT. It is failing the Class B limit (blue) by some margin.
Initial Isolation and Investigation
To investigate the emission radiation source (not the cause yet), I placed large clip on ferrite cores around the DC input cable and the battery output cable to reduce emissions directly from the cables.
This improves some of the frequencies but not all of them. If the radiation was entirely cable related then this would have dropped the emissions significantly. As it hasn’t, we can conclude that the majority of the emissions are coming from the PCB.
The three peaks we’ll focus on are 180MHz, 300MHz and 500MHz.
The location of the emissions for the 180MHz and 300MHz emissions was initially puzzling. Mostly it was centred around the drain of Q1. If we consider the operation of the circuit, Q1 is turned on permanently in boost mode with Q3 acting as the normal switching element and Q4 acting as a synchronous rectifier. Where is this switching noise coming from?
Those of you familiar with synchronous switching converter operation will be shouting at the screen right now. Of course, the answer is bootstrapping.
The high side N channel MOSFETs Q1 and Q4 need a gate voltage higher than their source voltage + their threshold voltage to turn on. In this kind of circuit, this voltage is derived from the switching node via a bootstrap circuit.
Even though Q1 is nominally on all the time it still needs to perform a switching operation with Q2 to charge up the bootstrap capacitor powering it’s gate driver circuit.
Checking the datasheet, this switching operation takes just 100ns. That’s very fast indeed and explains the source of our switching noise!
The same bootstrap operation is happening to provide the drive voltage for Q4 but because the boost node is continuously switching this voltage is being provided without such a short switching event.
Due to space constraints it wasn’t easy, but I managed to get the microscope out and modify the board to accept a small but high current ferrite bead in series with Q1 drain.
It didn’t take long to narrow down the 500MHz emissions to the boost output diode D1 with a large amount of ringing on the cathode.
The interesting thing about this diode is that it is only conducting for a very brief period in the dead-time between Q3 turning off and Q4 turning on. Dead time between these parts is set at 75ns, again a very short time period. Good for reducing switching losses, disadvantageous for EMC emission.
The part selected for this was a slightly electrically over-rated 40V 1A, SMB packaged part with a reasonable capacitance. Switching 1 amp of current through this part for only a brief period of time before shorting it out and discharging the diode capacitance was causing the ringing to occur.
A ferrite bead was added in series to damp this as the customer wasn’t too keen to head down the rabbit hole of investigating specifying a lower capacitance rated new diode or looking at whether the diode could have been removed altogether at the expense of slightly higher power dissipation in Q4.
Interestingly, this is what the emissions looked like with the diode removed but still with the lower frequency emissions present from the input transistor drain. Note the wideband reduction in emissions above 300MHz.
With both of the ferrite beads in place the emissions profile of the EUT was reduced to meet the Class B limits. With more time the peak at 160MHz could be investigated and further reduced but project time pressures and the customer understandably wanting a “good enough” result meant we concluded this investigation here.
DC/DC converters are often provide a challenging EMC opponent when it comes to radiated emissions. I was glad of the opportunity to work on this project and provide a successful result for the customer. This is the kind of work that I love.
The advantage of being an EMC-consultant-with-a-test-lab combined is that this kind of work can be compressed into hours of work rather than days/weeks oscillating between your lab and the test lab. Problems Fixed Fast!
I hope you found this piece useful, get in touch via the usual channels if you have any questions.
Ladies and gentlemen, I present this week’s episode of “Crimes Against Cables”
Example 1: “I had some leftover components to use”
I’ve seen plenty of interesting EMC “solutions” over the last several years to deal with radiation from cables.
A common one is to separate the shield ground from the signal ground with some combination of components (beads, capacitors, resistors). This approach appears to be particularly common on industrial touch screen display modules for some reason.
This is (in 99% of cases) a bad idea. I’m not sure what you are hoping to achieve by this and, probably, neither are you 😉
In fact I dedicated a small part of a recent talk to discussing grounds and grounding – you might want to check it out.
Example 2: How to Break a Shield
Another notable poor example was an otherwise well crafted piece of military equipment. Shielded connectors and cables all over, it looked like it would be survive some serious electromagnetic abuse (as anything being tested to MIL-STD-461 should).
This ends up being not only an emissions problem but an immunity one as well as the cables are just as capable of conducting noise into the shielded case.
This sort of thing can be solved with something like an EESeal type component or by a secondary external screen over the entire assembly.
Example 3: Plastic Fantastic
I’ve even seen ferrite cores that were just a moulded plastic lump to appear like cores. Maybe it was a “special” plastic? I never found out, it didn’t help the emissions either.
But this next one was a first even for me.
Example 4 – The Strangest Decision Yet
I was performing a full set of EMC tests to EN 55032 and EN 55035 for a customer. The product had a HDMI interface so obviously there were radiated emissions problems.
The first step as a diagnostic was to use some copper tape to make a connection between the connector shell and the metal back plate – the anodised chassis and EMI gasket material provided was not making a good contact.
This didn’t help so I buzzed the connection with the multimeter to make sure I had some continuity and… nothing.
No connection between the connector shell and PCB Ground.
OK, so there must be a capacitor in series with the shield connection. Fetch the capacitance meter and… 1.2pF.
The board designer had neglected to connect the shield of the HDMI to PCB Ground. It’s a new one for me!
The addition of copper foil to bridge the connector pins to nearby solved the emissions problem but left me wondering why someone thought that was a good idea.
I’m going to leave you with this closing thought:
I’ve yet to come across an EMC problem where floating or not connecting a shield ground has improved the situation.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2020/11/DSC_2492-scaled.jpg14402560James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2020-11-27 22:19:122020-11-27 22:22:55HDMI? More like HDM-WHY? Thoughts on Cable Shield Grounding
“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)
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.
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.
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.
There are two important characteristics about these emissions that show us where to look
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.
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.
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:
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.
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.
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.
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.
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.
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.
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.
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?
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?
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)
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…
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.
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.
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2018/09/yt_thumbnail.png7201280James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2018-09-30 14:31:302018-09-30 16:14:30Simple RF Current Transformer for EMC / EMI Investigation
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.
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! 😉
https://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2018/08/EMC-radiated-emissions-with-48MHz-harmonics-from-usb-to-serial.png8121025James Pawsonhttps://www.unit3compliance.co.uk/wordpress/wp-content/uploads/2017/01/unit3compliance_400x400.pngJames Pawson2018-08-28 14:39:022018-08-29 16:32:37RS-232 to USB Converters - EMC Problems Part Two
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”.
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.
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.
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.