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EMC radiated emissions problems from Riverdi LCD panel

Lawrence has been working on an Industrial (emissions Class A) pump product and has fixed an interesting EMC radiated emissions problem associated with the display panel.

Narrowband emissions caused by harmonics of a digital clock are nothing new when it comes to LCD panels and the signals that drive them. Inadequate filtering of these digital interfaces means that there is plenty of high frequency energy available to excite any parasitic antenna structure that we have created with these interconnected elements.

Normally the source of the emissions are the digital video signals from the CPU to the display. In this case, near field probing showed that the source of the noise was strongest around the display controller itself.

The first radiated emissions scan showed that there were emissions of a 60MHz clock signal present at around 300MHz and 660MHz (green = chamber background, red = measurement)

Here’s the display in question, a Riverdi RVT43 series touch screen for the Human Machine Interface (HMI) of the pump.

Lawrence took a closer look at the PCB with a contact probe and a spectrum analyser to find there was a Winbond QSPI flash running at around 60MHz.

Measuring each of the pins of the display cable (on the quad SMT resistor packs) showed significant levels of 60MHz noise on each trace, with the bottom most ones being the worst.

Looking at the circuit board we can clearly see where the problem lies:

We have a 60MHz clock trace (in red) running immediately adjacent to one of the signal traces leaving the PCB on an unshielded ribbon cable.

This is poor design practice on the part of the display manufacturer.

There is enough coupling between these traces (inductive and capacitive) for noise on the clock trace (source) to appear on the cable signal trace (victim). If we consider the fields around the traces in cross section through the PCB…

There is also the possibility of coupling inside the IC given that the output pins are immediately adjacent.

We can address the problem a number of ways

  1. Remove the noise at source by changing the PCB = not possible, this is a third party component. Changing the value of the series resistor packs could have worked in this case.
  2. Detune the antenna structure by changing the relationship of the two PCBs relative to each other = difficult to achieve, not guaranteed to work
  3. Adding common mode suppression on the cable to reduce the energy of the signal driving the parasitic antenna

Point #3 above was the only practical course of action.

A small ferrite core was placed over the cable and the results speak for themselves (green = before, red = after)

There is no compliance / EMC information / CE marking present on the display. But surely, as a piece of equipment or sub-assembly capable of causing radio interference, this falls under the scope of the EMC Directive and should be CE marked and assessed for EMC?

Yes, it should.

In this case, the host equipment manufacturer is having to deal with the poor hardware implementation by the display manufacturer.

Good work by Lawrence for getting to the bottom of this EMC problem.

Do I need to EMC test a pre-approved power supply? – EMC Explained

One of the most common questions we get asked when we send an EMC Test Plan / quotation to our customers is along the lines of:

 

“Our equipment is powered from a pre-approved CE marked power supply so we don’t need to do any AC mains EMC testing… right?”

 

If a power supply has already been EMC tested (if it has a CE or UKCA mark you would hope that this was the case) then it is a fair question – why should we retest it?

Adding AC mains specific tests into the EMC Test Plan adds time and therefore cost, something that some of our customers would like to avoid. For smaller businesses, the cost of assessment for EMC might be one of the largest external costs incurred on a project.

The main assumption driving this question is that EMC emissions – the noise that is coming out of the power supply and either back onto the AC mains or radiated from the power supply – is the only EMC problem we have to worry about. It’s the main one, but not the only one.

The pre-approved power supply will have been tested for immunity, but only the immunity performance of the power supply itself, not the equipment that it is powering.

Some noise will get through the power supply and into the equipment being powered. How does your product respond to this noise?

Also, how low are the AC mains conducted emissions from the power supply? Have you seen a test report? How reputable is the vendor?

Testing is the most reliable way to find out.

 

Our Recommendations

We generally recommend to our customers that they perform all of the applicable tests to the product.

(What, a test lab recommending testing? I’m shocked!).

Firstly, the tests are called up in the EMC standards, and for CE/UKCA marking, testing to a Harmonised Standard gets you a “Presumption of Conformity” to the requirements of the Directives – a pass without any further Risk Assessment or justification on your part.

Deciding not to perform the testing puts the responsibility on you to assess the remaining EMC risks. If you needed us to do this assessment for you or advise on it, the cost of a few hours of consultancy time would be equivalent to just doing the tests in the first place.

Secondly, EMC performance is often dictated by parasitic capacitances and inductances, component values that are not on the datasheet or intentionally designed into the product. Even knowing their magnitude does not give a good understanding of how they will interact. Testing allows us to measure their interaction under standardized conditions.

 

 

Risk Assessment Factors

As discussed above, our recommendation is always to perform testing on applicable ports, the AC mains port included.

If you are worried about costs or time taken for testing, then you might decide to omit some of the specific tests. The below table outlines some of the factors you may wish to consider when making this decision.

The more items that apply from the Risk Increasing Factors column, the less strong your argument becomes for not carrying out testing.

 

Risk Reducing Factors Risk Increasing Factors
Class II power supply (un-earthed)

 

Class I power supply (earthed)

Especially if the DC negative of the power supply output is connected to Protective Earth in the system.

Power supply comes from reputable vendor (e.g. Meanwell, XP Power, Recom, Traco, TDK Lambda, Puls, etc) Power supply comes from cheap or from far east supplier
Power supply external to product Power supply internal to product
No analogue or sensitive circuitry Analogue circuitry e.g. audio, 0-10V I/O, 4-20mA I/O

Sensitive, low level signals e.g. thermocouple, RTD

No other long (>3m) cables connected to equipment One or more long (>3m) cables connected to equipment
Main use in Basic (residential, commercial) EM environment Flexible use, could be used in Light Industrial or Industrial EM environments

 

If you are at all unsure then you should test the AC mains port with your intended production power supply.

For the ultimate in performance, or if the equipment is for flexible use (could be powered from an AC/DC supply or from a distributed DC power supply) then we would recommend treating the DC power input to your product as a signal port with a length greater than 3m.

This would then call up Conducted RF Immunity (EN 61000-4-6) and Electrical Fast Transient (EFT, EN 61000-4-4) testing to the power port at the appropriate levels for the end EM environment (e.g. Basic or Industrial)

One step further would be to apply line-to-line and line-to-earth surges to the DC input, assuming that the design already contains a transient surge voltage suppressing element like a TVS diode or an MOV.

Let’s take a look at some of the technical justification behind the selection of these items.

 

AC Mains Port vs DC Power Port

If you typically derive your equipment power from an AC mains power supply, then it is unlikely that you will fall under the DC Power Port classification.

The term DC Power Port in EMC terms means a very specific classification of port. We discuss this in some length in this article.

 

Power supplies do not always meet the regulations

A scenario that we have experienced on several occasions: the power supplies that end up with our customers or in our test lab are not the same as the ones in the manufacturer supplied EMC test report.

 

Another customer had similar problems on  power supply that they had received samples of in that the EMC performance varied wildly. In this case the clue was that the weight of the two samples was significantly different.

 

 

These power supplies were almost identical on the outside but significantly different on the inside. Same manufacturer and model number, different components. Imagine the conversation:

“I’d like to order some HM-A132 power supplies please”

“Certainly sir, which ones?”

“Erm…”

 

This is mostly related to cheaper power supplies sourced from China. We often see significantly different results to those shown in the manufacturer test report.

The worrying thing is if changes like this are being made on the basis of EMC, what changes are being made that affect Electrical Safety that are going unchecked? We can check that for you as well.

 

Cable Routing

If your power supply is integrated into your equipment then there is the possibility of noise on the AC mains cable coupling onto other nearby cables.

It is also possible for noise to couple (both to and from) components connected to the AC mains and internal system components. This could be an emissions (noise getting out) or an immunity (noise getting in) risk.

 

This is particularly likely if you are using slotted trunking and mixing AC mains cabling in with other cables.

 

 

 

This is less important for an external power supply like a laptop type charger or a plug top power supply as the AC mains cable remains outside of the equipment enclosure.

 

 

Power Supply Common Mode Impedance

Electrical noise inevitably gets coupled onto the AC mains bus. Normally this noise is coupled onto the AC mains Common Mode. This means all the lines together in relation to a high frequency “ground” reference plane.

The noise current through the power supply and equipment will flow something like this:

The noise reaching the equipment will have been attenuated by the Common Mode impedance of the power supply and the currents diverted through the parasitic capacitance of the power supply relative to the HF ground reference plane used in the tests.

Crucially, some noise still gets through to the power supply and will flow through the product. The magnitude of this current can be estimated or measured but relies on electrical parameters that are not on the power supply datasheet.

It is this noise current that we are interested in. How does it affect your product? The only way to find out is to perform testing.

 

Class I vs Class II Power Supply CM Impedance

The construction of a typical switch mode AC/DC power supply is broadly similar across a wide range of topologies. One of the main EMC variations results from if the power supply is Class I (earthed) or Class II (unearthed).

 

Class II

A Class II power supply relies on Double or Reinforced insulation between Live parts and user accessible secondary low voltage parts for Electrical Safety. There is no connection to Protective Earth. This kind of power supply is usually identifiable by:

  • the square-in-a-square double insulation symbol (IEC 60417 symbol # 5172)
  • a plastic earth pin on a UK mains plug (technical name is an ISOD or Insulated Shutter Opening Device)
  • An IEC C8 “figure-8” AC mains inlet socket with just two pins

 

Looking at the typical internal structure of a Class II AC/DC SMPS we can see that the components providing Common Mode noise attenuation are

  • the inductive common mode filter (Lcm)
  • the components across the safety isolation barrier, transformer Tx and class Y capacitor Cy

The value of parasitic parallel capacitance of the choke or transformer (or wanted series capacitance of Cy) will reduce the impedance ( Xc = 1 / [ 2 * pi * f * C ] ) and allow more noise current to flow at higher frequencies.

This capacitance is usually a low value to prevent too high a touch / leakage current to flow which would compromise Electrical Safety.

However, at EMC frequencies of MHz and higher this presents a much lower impedance allowing noise currents to flow through the cable.

 

block diagram of a class II power supply showing EMC immunity noise current through the power supply

 

Because current always flows in a loop, and because current always returns to the source, to close this common mode current loop we need to have return currents flowing. We usually think of these coupling capacitively onto a nearby metallic element like a nearby metal structure.

In the test lab we simulate this with a nearby metal plate but in real life this could take a number of forms (building steelwork, conductive cable trays, other wiring).

 

 

Class I (Or Class II with Functional Earth) Power Supplies

With a Class I power supply, the Protective Earth is connected to accessible metalwork for Electrical Safety reasons (prevention of electric shock). Basic insulation (or higher) is required between the live parts and user accessible secondary parts.

Possibly the protective Earth is also connected to DC negative somewhere in the system as well.

A Class II with Functional Earth power supply is similar from an EMC point of view but very different from an Electrical Safety point of view. In this case, the Earth is connected for functional reasons (reducing noise or EMC emissions) but the power supply still relies on Double or Reinforced insulation for safety.

This isn’t a very common power supply topology choice, so I was surprised to see it marked on my laptop charger power supply.

 

 

In both cases, when we apply common mode noise to the AC mains input (L+N+E) then the Protective Earth conductor allows the noise to bypass the common mode impedance of the power supply. It is for this reason that we view the use of a Class I earthed power supply as a higher EMC risk for immunity reasons.

 

block diagram of a class I power supply showing EMC immunity noise current through the power supply

 

 

How this noise couples into the rest of the equipment, its magnitude, and how it affects it depends massively on the construction of the equipment. Again, testing is the best way to determine this.

 

Conclusion

Power supplies and the equipment they power are not perfect and can have varying EMC performance depending on how you connect them and how the equipment is designed.

It isn’t always easy to estimate how likely EMC issues are, even for experienced engineers and problem like us at Unit 3 Compliance. It is for this reason that we would always recommend testing to characterize the unknown EMC performance.

If you do decide to omit some testing, then the Risk Reducing or Increasing Factors above should help with that decision.

Again, we hope that this guide was useful to you in some way. Get in touch with us if you have any thoughts, questions, observations, or (obviously) a need for EMC or Electrical Safety testing.

All the best!

 

 

 

2.4GHz Wi-Fi – Effect of Settings on Second Harmonic at 4.8GHz

When using a 2.4GHz radio inside a product, one of the most common radiated emissions problems observed is the second harmonic at 4.8GHz approaching or exceeding the limit line.

This can be caused by a nearby parasitic antenna structure (e.g. metalwork, PCB cutouts, wiring looms) or by using high output powers in the Wi-Fi radio.

In this case, the product was a handheld battery powered device with a uBlox Wi-Fi module.

The customer wanted to try different settings to judge the effect on the 4.8GHz harmonic to remedy a test failure at another lab.

Radiated emissions being measured to EN 55032 Class A in our Fully Anechoic Chamber.

 

Takeaway

The important takeaway from this exercise (and something that we will be doing in future) is to measure the 2nd harmonic radiated emissions of Wi-Fi

— at both channel 1 and channel 11 (6dB difference in this case)
— and at the lowest modulation / data rate (7dB difference in this case)

Cumulatively, this could make up to *13dB* of difference based on these figures.

 

 

Experiment 1) Add a 2.4GHz Notch Filter

The first thing we would do when measuring an RF emission second harmonic is to evaluate how much is being caused by overload of the measurement system (preamp, spectrum analyser). We add a -30dB 2.4GHz notch filter in series with the antenna inside the chamber before preamp or any other active measurement equipment.

This removes the RF carrier from the input of the measurement system, preventing overload. The overall insertion loss at 4.8GHz is in the order of 1.5dB

 

Wi-Fi Channel

Modulation 2.4GHz Notch Filter Output Power Polarisation 2nd Harmonic Margin (to Class A limits) (dB)

1

1Mbps/20MHz No 6dBm V

-3.2

1 1Mbps/20MHz Yes 6dBm V

-7.52

 

Accounting for the 1.5dB reduction in the filter, we reduce the 2nd harmonic by 3dB. So what we are measuring is mostly real and not an artefact of the measurement system.

 

Experiment 2) Increase Output Power

This was to see at what power level the 2nd harmonic would meet the limit.

The non linear characteristics of the curve certainly point towards saturation of the power amplifier part of the Wi-Fi module RF output.

One might be tempted to pick an output power of 14dBm as having sufficient margin. However, given likely variations in operating temperature, supply voltage, tolerance across units, etc. an operating point of 12 or 13dBm would be more prudent.

 

Wi-Fi Channel Modulation 2.4GHz Notch Filter Output Power Polarisation 2nd Harmonic Margin (to Class A limits) (dB)
1 1Mbps/20MHz No 6dBm V -3.2
1 1Mbps/20MHz Yes 6dBm V -7.52
1 1Mbps/20MHz Yes 11dBm V -7.18
1 1Mbps/20MHz Yes 13dBm V -9
1 1Mbps/20MHz Yes 14dBm V -3.1
1 1Mbps/20MHz Yes 15dBm V +3.44
1 1Mbps/20MHz Yes 16dBm V +6.18

 

 

This graph has an interesting characteristic. Above 13dBm we suspect that the amplifier in the Wi-Fi device is starting to saturate causing additional second harmonic products.

Further testing took place at 14dBm to see if it increased or decreased the measured level as this was located centrally on the steep part of the curve.

 

Experiment 3) Changing the signal bandwidth and modulation scheme.

Lower data rate / bandwidth signals appear to have the highest occurrences of 2nd harmonic emissions. Very little difference moving from 20MHz to 40MHz bandwidth.

 

Wi-Fi Channel Modulation 2.4GHz Notch Filter Output Power Polarisation 2nd Harmonic Margin (to Class A limits) (dB)
1 1Mbps/20MHz Yes 14dBm V -3.1
1 6Mbps/20MHz Yes 14dBm V -6.51
1 12Mbps/20MHz Yes 14dBm V -10.15
1 54Mbps/20MHz Yes 14dBm V -10.7
1 54Mbps/40MHz Yes 14dBm V -10.1

 

 

 

Experiment 4) Change Wi-Fi Channel / Frequency

The most interesting result was changing the Wi-Fi channel frequency. Increasing it from channel 1 to channel 11 caused the second harmonic emissions to drop by 6dB.

 

Wi-Fi Channel Modulation 2.4GHz Notch Filter Output Power Polarisation 2nd Harmonic Margin (to Class A limits) (dB)
1 1Mbps/20MHz Yes 14dBm V -3.1
3 1Mbps/20MHz Yes 14dBm V -5.5
6 1Mbps/20MHz Yes 14dBm V -7.56
9 1Mbps/20MHz Yes 14dBm V -9.37
11 1Mbps/20MHz Yes 14dBm V -9.5

 

 

 

 

meanwell power supply comparative radiated emissions

Meanwell Power Supply Radiated Emissions Investigation

We’ve been doing pre-compliance scans on a customer’s product and helping them overcome some interesting radiated emissions problems (spoiler, 3rd party display module. Again.)

Now that it is back in the lab for it’s final EMC measurements we suddenly found that we were measuring an extra 10dB of noise at 170MHz.

Hang on… it was passing during the pre-compliance measurements last month… what is going on?

During the pre-compliance measurements, we only saw this peak when we were powering the EUT from the provided open frame mains power supply. So we removed it for some investigations.

Using our Tekbox TBCP2-750 current probe and Signal Hound BB60C spectrum analyser, we measured all of the cables connected to the power supply.

 

On the AC mains supply:

current probe on ac mains input

On the DC power output cables:

current probe on dc power output

and on the 12V auxiliary fan power cable

current probe on 12v aux fan output

 

What we measured was a bit of a surprise:

meanwell power supply comparative radiated emissions

 

Of all the cables we expected to have a problem with, the low power 12V fan cable was not our first candidate. It looked to be carrying the most noise at 170MHz so we did what every good EMC engineer does – put a ferrite on it!

 

 

Now it meets Class A with a 5dB margin at that frequency.

Upon further investigation, a second fan had appeared inside the equipment since our pre-compliance measurements. The engineer had mentioned improving the temperatures within the product but we hadn’t opened it up to verify if any changes had been made.

The cable routing for the new fan was undefined, allowing it to lie across the power supply, or next to other components depending on how it was assembled. This appeared to be the cause of variability that we had observed in our testing.

 

Takeaways

One of the key rules of EMC troubleshooting is to change only one thing at once, and be careful that you are only changing one thing. Reassembling the unit with different fan cable position accounted for some of the variability in emissions performance.

Don’t assume that just because you are using a pre-approved component that it will automatically pass when integrated into your system. Having worked with many Meanwell power supplies of all different flavours over the years this is only the second time we’ve had any significant issues with one.

 

Bye for now,

James

 

 

 

MPS Presentation on DC/DC Converter Myths

I wanted to share this excellent and useful presentation by Francesc Estragués Matamala from Monolithic Power Systems.

This addresses many myths about DC/DC converter layout and I love tackling EMC myths!

I’ve been wanting to do a study like this for a while now so thanks to MPS for doing this.

Make the time to watch this.

 

 

I Will Happily Spend Your Equipment Budget…

 

 

Customers sometimes ask me “what EMC pre-compliance equipment should we buy?”

My reply is that I’m more than happy to help them spend your equipment budget!

Here’s an email that I sent to a customer recently when they were asking for feedback on some test equipment that had been proposed to them

 

A good spectrum analyser is pretty indispensable when it comes to wrestling with EMC issues. The Siglent ones (available from I4E and Telonic) are pretty damn good for the money, I’d buy one if I was in the market.

Near field probes can be used to narrow down the emissions source pretty effectively. Either the Tekbox ones from Telonic or the Beehive ones from Farnell are pretty good.

I have just written a free ebook on the subject of near field probing which might be of interest.

Another good addition is a current probe like the one I brought during my visit. This lets you easily characterise emissions on cables. Add an attenuator set to protect the spectrum analyser input.

Challenges with on site pre-compliance measurements

  • Dealing with background noise (near field probes and current probes are pretty resistant)
  • Relating levels it to a formal measurement standard (not possible)
  • Interpreting the results and figuring out what to do about it (expert level!)

If you decide to go down the road of getting your own equipment I’d be happy to come up and do a day with you running through measurement setups, tips and tricks if you think that would be helpful.

Alternatively, if you want to make some antenna based on-site radiated measurements I can come up and do a day with you with our spectrum analyser and portable antennae.

 

 

 

 

One Ferrite Is Not Enough

This would be a great Bond film title…

“So Blofeld, do you expect me to talk?”

“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:

  1. 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.
  2. 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!

 

Graphical Guide to EMC: Near Field Probing (free eBook)

 

You can download our free eBook on Near Field Probing here

I have a love / hate relationship with textbooks.

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 (and slept 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 + 5/8 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 our free eBook on Near Field Probing here

 

 

 

I’ve released this under the Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license. This means you can share or adapt this work but you must provide a credit / link back to the original source (here). Any adapted work must be shared with the same licence terms.

 

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!

Thanks and happy probing!

James

 

 

self interference demo USB3 and 2.4GHz

2.4GHz Intra-System (or Self/Platform) Interference Demonstration

In this blog we are going to take a short look at noise and interference in the 2.4GHz band. Our example victim is a Zigbee controller and the sources are nearby USB3.0 devices and Wi-Fi sources.

 

Background

One of our customers makes these rather useful USB Zigbee Coordinator sticks, frequently used for controlling smart home or IoT devices like light bulbs.

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.

 

Demonstration 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.

 

self interference demo USB3 and 2.4GHz

 

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.

 

spectrum of 2.4GHz band showing ambient noise, SSD noise and Wi-Fi emission

 

Experiment Conclusions

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.

 

Solutions

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.

shiny usb cable vs boring usb cable

 

 

Hope this was useful! See you soon.

James

 

 

 

RCWL-0516 - board image from github.com-jdesbonnet

Compliance Assessment of a RWCL-0516 Doppler Radar Motion Detector

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.

RCWL-0516 - board image from github.com-jdesbonnet

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.

RCWL-0516 - radiated emissions Class B domestic

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
  • No reference to technical standards used to assess the device to the Radio Equipment Directive
  • At present there are no radio standards published by ETSI for the use of this 3.1GHz band for this kind of application in the EU or UK.
  • This document from CEPT on the use of Short Range Devices gives more details about what radio bands can be used
  • 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.

RCWL-0516 - CEPT radio band table

  • Even when taking this table into account, this band is only for UWB Location Tracking Systems.

RCWL-0516 - CEPT radio band table part 2

  • 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.
  • From a regulatory standpoint, these devices should not be touched with a barge pole
  • 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.

 

Summary

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.