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ram cage being removed from a 2018 mac mini

Apple Multi-Purpose EMC/EMI Shielding

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

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

 

Even the EMI shielding solutions are stylish

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

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

Let’s discuss these below.

ram cage being removed from a 2018 mac mini

Image from iFixit

 

Why is the screening can so important?

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

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

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

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

inside shot of mac mini case with component analysis

Original image courtesy of iFixit, markup by author

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

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

 

Noisy Neighbours.

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

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

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

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

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

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

 

Improve Performance? The Can Can.

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

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

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

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

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

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

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

 

The Last Line Of Defence

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

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

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

 

 

IoT EMC Radiated Emissions Investigations

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

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

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

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

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

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

IoT module - emc radiated emissions analysis

Let’s take a wander around the device.

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

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

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

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

 

Conclusion

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

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

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

 

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

 

Antenna Miniaturisation

An article just published on Nature Communications covers some interesting and quite exciting developments in antenna miniaturisation.

 

Overview

Electrical antennae rely on being resonant or partly resonant with the electromagnetic wave at the frequency of interest. For a dipole, a length of lambda / 4 is ideal but antennae can be designed with electrical lengths down to lambda / 10. The technique of antenna construction shown in this research means that effective antennae can be constructed that work at electrical lengths of nearly lambda / 600.

If you take a 950MHz (mobile phone low band frequencies) full dipole it will have a length = c / (f * 2) = 16cm. With this new technology, this antenna length could come down to sub 1cm distances. This decrease by over a factor of 10 is highly significant and with the application to sub-resonant antenna designs, further decreases in size could reasonably be expected.

 

Technology

The technology itself is very clever, sandwiching a layer of piezoelectric material (voltage to mechanical movement) with a layer of ferromagnetic material to form what the paper refers to as a “ME heterostructure” (I prefer sandwich… mmmm… sandwich). The magnetic part of an incident EM wave causes the ferromagnetic layer to change shape in response. In turn, this makes the bonded piezoelectric layer change shape, creating a voltage on the output terminals. The process of reception is now mechanical rather than electronic with the tuning of the antenna primarily performed by selecting the materials based on their mechanical properties, thereby tuning the mechanical resonant frequency.

Similarly, applying a voltage to the piezoelectric material will change its shape, causing the shape of the

It’s worth taking a step back to point out that you can perform a similar trick with a conventional antenna by increasing the dielectric constant of the surrounding material. This changes the speed of light within this medium which is dictated by the Velocity Factor (VF) = 1 / sqrt [Er] (where Er is the dielectric constant or relative permittivity). This is why placing your antenna closely coupled to a plastic enclosure can change the effective frequency. Two downsides that make this approach less useful for antenna miniaturisation:

  1. The range of dielectric constants of insulating materials. FR4 PCB material is typically around 4, silicon and alumina can be in the 11-12 region. Higher values for liquids or more exotic materials do exist.
  2. The square root term around the dielectric constant causing the reduction in VF to become less significant with increasing Er

The mechanical “stiffness” of the new antenna (analogous to dielectric constant) is suitably high that the resonant wavelength is much smaller in the materials used leading the the significantly smaller antennae producible using this technique.

 

CMOS Process

What is even more exciting is that these new antennae are produced using a CMOS process (detailed in the supplementary material).

This means that integration of the antenna and receiver could be integrated onto the same silicon die in future. Not only would this make things ridiculously compact but it would also allow the receiver circuitry to be placed electrically right next to the antenna which could reduce coupling of noise and improve sensitivity.

It also gives the scope, as mentioned in the paper, of being able to print arrays of antennae on the same die. This could be used to make compact phased arrays, wideband arrays or just the key antennae for different frequencies required for a mobile communications device.

IoT (the favourite industry buzzword at present) solutions will get smaller, cheaper and easier to integrate, perhaps leading to a further connected-ness revolution. The potential impact is fascinating.

 

Magnetic Coupling and the Near Field

Because these antenna operate on the magnetic component of the EM wave, it should make them more efficient when operating in the near field.

Original image can be found here.

The near field is the distance between antenna and source when the ratio of distance to wavelength is below the ratio of 1 / 2*pi (see above chart). In this region, the impedances of the magnetic and electric fields are not related by the impedance of free space (approx 377 ohms). The field from a magnetic dipole decays with the cube of the distance (not the square) so being able to place a small magnetically sensitive resonant antenna closer to the magnetic field source provides a new method for sensing small currents right next to the source.

 

Downsides?

The decrease in size could lead to problems in the capacity to handle higher transmit powers or currents. Having said that, the thermal coupling of the antenna to a solid material should be good meaning the thermal resistance of the antenna to ambient could be both quite low and controllable.

Magnetic materials have other characteristics such as saturation and hysterisis so it will be interesting to see how it handles large DC or low frequency AC magnetic fields caused by electrical power wiring. I wonder if this will mean it gets classified as a magnetically sensitive component and starts to fall under the remit of IEC 61000-4-8 testing for such things.

Having worked on designs involving high voltage differential piezoelectric transformers below, I know that they require careful mechanical handling as they will chip, crack and break if treated roughly (although, probably just as much as ferrite of the same dimensions). Conventional metallic structure antennae are generally quite robust or can be made so. I believe the small mass and the encapsulation of these antennae in a plastic or ceramic package could well solve many of these issues.

This also presents the possibility of mechanical interference to an electrical signal. I don’t have a handle on what sort of high frequency mechanical vibrations might exist in the real world and I can imagine their energy would be quite low at the really high frequencies with elasticity of materials starting to take over. I can imagine trying to debug an RF sensitivity issue with a stethoscope for a change!

The technology website Futurism speculates that these could form the basis of a brain-computer interface. Needless to say, the EMC immunity applications of such a technology, especially in our increasingly “EM-dense” environment would be of massive concern. Perhaps we will be seeing the imminent return of the tin-foil hat as a genuine reason for keeping the government out of your brain!

You can read the full article here.

Header image taken from the original article.

NFC Antenna Optimisation

We’ve recently been working on optimising an Near Field Communications (NFC) card reader antenna using our skills in wireless integration and antenna design. The customer had an existing system and a new requirement to mount the read antenna at the end of a cable away from the NFC reader mainboard.

This involved us working on the original system by modifying the existing antenna to the required inductance, implementing and tuning an impedance matching network and trialling different cable types. Remember, the rules for tuning the read antenna are different to those for the receive antenna.

Despite the design challenges we were able to achieve a card read distance up to 30mm away from the antenna and provide a robust solution for mounting on different surface materials. The customer was happy and we look forward to seeing the product out in the field!