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Pocket Near Field Probe Boards (R5) – Now with 100% more layers!

Hello and welcome to Unit 3 Compliance.

We test your products for EMC and Electrical Safety, fix them if they don’t pass, and help you with all that boring standards and documentation stuff.

We also provide customised design for EMC training via Think EMC.

Hot off the press are the revision 5 of our Pocket Near Field Probe kits. Better than revision 4? “How is that possible?” I hear you ask…

New Features

  • Now on a 4 layer PCB (100% more layers!)
  • All sensing and signal traces are enclosed within ground plane sections on the outside of the PCB – preventing pickup from stray fields and reduces effects of fingers
  • Removed snap off sections
  • Better silkscreen markings
  • More formulae! (ok, just 3)

Download our Guide

But first things first – if you are new to Near Field Probing then you simply must check out our free eBook that has everything you needed to know (and probably a lot that you didn’t) about the art of near field probing to help solve EMC problems.

* Click here to download *

Assembly Instructions

Refer to the instructions in our Revision 4 guide. It’s exactly the same, only we’ve removed the snap off sections.

We hope you enjoy using it!

 

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.

Pocket Near Field Probe Boards (R4)

Hello and welcome to Unit 3 Compliance.

We test your products for EMC and Electrical Safety, and fix them if they don’t pass.

We also provide customised EMC training via Think EMC.

If you have picked up one of our new Pocket Near Field Probe kits from us then, oh my goodness, we hope you like it as it is one of our best yet. We’ve added cool new features and some useful EMC reference formulae as well as the near field tools from the original.

Download our Guide

But first things first – if you are new to Near Field Probing then you simply must check out our free eBook that has everything you needed to know (and probably a lot that you didn’t) about the art of near field probing to help solve EMC problems.

* Click here to download it *

 

What’s In This Probe?

Get started assembling by snapping off the two parts shown.

The first part is our contact details and, on the back, a quick reference of useful EMC formulae.

The bit from the top is a new addition: an SMA spanner (careful with that torque Eugene) and a key for opening Wurth Elektronik snap on ferrite cores.

You can then assemble the rest as described below.

 

Assembly Guide

Components required:

0805 resistors (10:1 voltage probe)

R1 = 470R and R2 = 10k, gives a 450 ohm parallel combination. This is required for the 10:1 divider into a 50 ohm input.

0805 capacitor (10:1 voltage probe)

C1 = 220pF to 2.2nF, C0G/NPO dielectric, 50V

This AC couples the signal from the probe into the measuring instrument. Many spectrum analysers have a 0VDC requirement on their RF input.

The actual value isn’t too important at the frequencies we are interested in.

Metal pin for contact probe

For the contact probe pin we recommend using a male pin header from a 2.54mm pitch connector strip and filing the end to sharp point with a metal file.

These parts are available from all major component distributors and from eBay again.

We also recommend securing this in place with a generous blob of 2-part epoxy adhesive to provide some additional mechanical strength.

Edge mount RF connectors

The 1.6mm thick PCB is designed to take SMA edge mount female connectors to connect to an SMA-SMA RF cable.

These connectors are available from a wide range of sources but most readily and cheaply from eBay

You can solder a coaxial cable onto the pads at the expense of ease of swapping the connection between probes.

Ferrite cores (current transformer adaptor)

The current transformer adaptor was designed to use the excellent Wurth Elektronik 742715x series of ferrite cores which are available in different sizes.

If you do get one, tie the two-pronged plastic unlocking key to the probe with a bit of string so that you don’t lose it!

An M3 screw and nut will secure the core to the board, but again a blob of two-part epoxy adhesive or some hot melt glue will do the trick.

Experiment with the number of turns but we generally use between 5 and 10 as a starting point.

 

Finished Product

Here are our probes that we use in the lab for all kinds of EMC problem solving:

 

And if you really know what you are doing with these probe boards, there’s a secret power user Easter egg mode available.

Tea up!

 

Mug is from Mike’s Electric Stuff

 

 

 

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

 

 

 

conductive contamination underneath surface mount isolated power supply causing line to earth surge failure(marked up photo)

Surge Test Failure Due to PCB Manufacturing Process

We recently had a piece of customer equipment fail the IEC 61000-4-5 surge test at 2kV line-to-earth. There was a loud crack of an electrical arc forming, the unit stopped responding to communications and was making a hissing/squealing noise.

To give it the appropriate technical term, this was “A Bad Thing”.

Using the thermal camera we quickly found several hot components all on the 3V3 supply line that we supposed had been damaged by the surge. The hissing noise was the DC/DC converter in a cycle of burst mode trying to supply too much current before shutting down.

However these were all secondary side components on the isolated part of the system. How did the surge get across the safety barrier? The designer was using correctly rated parts and the PCB creepage distances were dimensioned correctly.

As part of the fault diagnosis process, we used our hot air solder rework tools to remove one of the isolated power supplies providing a low voltage supply to the AC mains monitoring circuitry. Underneath we found this:

 

conductive contamination underneath surface mount isolated power supply causing line to earth surge failure(marked up photo)

 

The samples had been hand soldered by the customer, unfortunately leaving a large amount of solder paste underneath the power supply.

Whilst this was not a short circuit across the safety barrier it did reduce the creepage distance significantly. When a 2kV surge (1.2/50us, 12 ohms) was applied from AC mains to earthed secondary this pollution was enough to cause an arc to form and into the 3V3 supply pin (centre right).

This voltage was enough to fry several components on the 3V3 line, rendering the board inoperative.

 

Lessons Learned

  • Hand soldering prototypes is OK provided you take great care in the process and cleaning the board afterwards
  • Professionally manufactured boards will generally avoid this issue
  • Apply a line-to-earth safety test on your AC mains powered products to check your samples
  • We are going to start a policy of performing a line-to-earth safety test on all AC mains powered products coming into the lab for testing from now on to try and catch problems like this.

 

 

V2 pocket EMC debug probe PCB - near field probe set - board front

Pocket EMC Debug Probe V2

A Bag of Water.

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.

radiated emissions plot

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.

 

Caveats inbound

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!

James

Low Frequency, Common Mode, Conducted Emissions

Here is an interesting problem I had working on piece of industrial equipment. The customer had some conducted emissions failures at another EMC lab and needed some help resolving them.

The lessons from fixing this problem was that the first thought is not always the correct one, and that sometimes, all you need is a bit of green-and-yellow earth wire!

 

Outline

A block diagram of the system is shown below with the major components shaded.

An industrial power supply feeds power to the controller (a custom PCB connected to a Raspberry Pi) and to the power measurement board (measures the power consumed by the load).

 

 

Conducted emissions on both the Ethernet port and the AC mains port on the power measurement board were both dominated by a low frequency hump around 700kHz.

 

AC Mains

Ethernet

Notice how the shape or profile of the emissions is almost identical. To my mind, this points towards a single component in the system causing the same noise to be seen everywhere.

 

Simplify First

The first thing I wanted to do was to simplify the test setup as much as possible. I replaced the industrial power supply (often designed for Class A emissions performance) with my trusty Thandar TS3022S adjustable linear bench supply.

The idea here was to eliminate the industrial power supply from my inquiries.

 

 

Wow, what a big difference!

 

So the conclusion here is that the industrial power supply DC output is very noisy, that this noise is propagating through the system, and manifesting as conducted emissions on the outputs via a variety of coupling paths.

 

Differential Mode Filtering

Because conducted emissions noise in this lower frequency range tends to be differential in nature (+ve relative to -ve), my first thought was to implement a differential mode filter on the output of the power supply.

 

 

I’ve got a little filter prototype board that I use in situations like this. This pi filter was made up from two Panasonic FC series 470uF, 25V on either side of a Wurth 33uH iron powder inductor.

 

 

Unfortunately it did nothing to the emissions!

 

Could it be Common Mode?

This sounds like a obvious question to ask in hindsight. Most EMC problems are common mode in nature, I’m just used to thinking about LF conducted emissions as a differential mode problem.

Let’s try a common mode mains filter on the output of the power supply to see if this is indeed the case.

 

 

That’s much better! It looks like the problem was common mode noise after all.

 

This Time It Was Actually A Good Idea…

Common mode noise in this instance is current on both the DC output lines together. But, as I point out in one of my talks, current flows in a loop and always returns to the source. So where is this common mode current returning to? What is it’s reference?

Our common mode emissions measurements are being made in relation to the metalwork of our screened room test setup which is connected to the AC mains Protective Earth (PE).

The AC mains line to each LISN contains a PE connection and, inside the LISN, this is connected directly to the floor of the chamber.

Logically then, connecting the DC negative to the PE on the power supply will provide a shorter path for this common mode noise from the power supply.

 

 

Will this have the desired effect on emissions?

Yes. Yes it does.

AC Mains

Ethernet

 

Conclusion

Ooooooh, bloomin’ common mode noise. Not just for the higher frequencies but lower ones too!

This was a fun half day project fixing this particular problem. Much nicer to be able to recommend a low cost cable assembly than £$€ 20 worth of filter block.

If you’ve got any EMC problems then give me a call, I’d be happy to help.