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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

 

 

 

 

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?