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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

 

 

 

 

Choosing EMC/Radio Standards for CE/UKCA – Generic vs Specific

A short post prompted by a (summarised) request from a customer:

 

We’d like to test to the following standards for our CE/UKCA marking

– EN 61326-1 (Class B emissions, Industrial immunity)
– EN 61000-6-2 (Industrial Level Immunity)
– EN 61000-6-3 (Class B Emissions)

 

This customer is very compliance conscious, as their products end up in all kinds of harsh and hazardous environments where they are protecting the health and safety (and lives in many cases) of their customers.

As such, it is understandable that they want to “throw the kitchen sink” at the EMC performance. Selecting Class B emissions and industrial immunity is a great way of demonstrating the robustness of your product in a wide range of electromagnetic environments.

So, why not quote all of the standards on the Declaration of Conformity (DoC)?

 

CE and UKCA

This article was originally written with CE in mind. It also applies to UKCA, just replace “Harmonised” with “Designated” as far as the standards go and you’ll be fine.

 

Guidance is Available

Thankfully the European Commission has published guidance on selecting Harmonised EMC and Radio standards for assessing the product to.

In each of these standards, a primacy or order of application, is given to the Harmonised Standards.

 

Guide for the EMCD (Directive 2014/30/EU)

4.3.2.2 Relevant harmonised standards

The selection of the relevant harmonised standards is the responsibility of the manufacturer.
When the manufacturer chooses to apply harmonised standards he shall select them in the following precedence order:

– Product-specific standards (if available)
– Product family standards (if available)
– Generic standards

Product-specific (family) standards are those written by ESO’s taking into account the environment, operating and loading conditions of the equipment and are considered the best to demonstrate to compliance to the Directive.

 

An example of a product specific standard would be EN 61326-2-6Electrical equipment for measurement, control and laboratory use – EMC requirements – Part 2-6: Particular requirements – In vitro diagnostic (IVD) medical equipment (IEC 61326-2-6:2012)”

These product specific standards often refer back to the root family standard, EN 61326-1 in this case.

Only if the manufacturer’s equipment does not fall into a product standard should the generic standards be applied.

 

Guide to the Radio Equipment Directive 2014/53/EU

5.2 Generic harmonised standards vs product specific harmonised standard

A manufacturer which has the intention to apply a harmonised standard for the conformity assessment of its products, has to apply in priority the product specific harmonised standard and only if this one is not available, the generic one, in order to benefit of presumption of conformity with the essential requirements of the RED.

 

Applying Multiple Standards

There are cases where applying several different Harmonised Standards could be the correct thing to do.

For example, if the equipment is a piece of measurement equipment that incorporates a lot of IT functionality (networking, data storage, PC control) then the manufacturer could decide to assess against EN 61326-1 for laboratory equipment and against EN 55032 for IT equipment. Both standards would appear in the test report and on the DoC.

 

Check Annex ZZ

One of the commonly overlooked Annexes (Annecies? Annecii?) is this one at the start of the standard. This details what Essential Requirements from the Directive are being covered by the standard.

Important: not all standards cover all Essential Requirements. You must check Annex ZZ carefully against them.

If you end up needing to apply more than one Harmonised Standard to a product to cover all of the Essential Requirements then you should state this on your Declaration of Conformity.

 

Presumption of Conformity

Remember that using Harmonised Standards (or Designated Standards for UKCA) gives you a “Presumption of Conformity” without further requirement to demonstrate compliance with the relevant directives/laws.

As this interesting piece on kan.de notes:

 

“Ultimately, the presumption of conformity is no more than a reversal of the burden of proof. This means that a product complying with the relevant [harmonised] standards may be challenged, for example by the market surveillance authority, only if actual evidence can be produced that the manufacturer has violated the requirements of the directives.”

 

Annex ZZ of a Harmonised Standard is your friend when it comes to understanding this link between the standards and the directives.

 

When the DoC Doesn’t Quite Cover It

This example of EN 61326-1 illustrates one of the problems of applying a Harmonised Standard that has multiple levels within it.

In this case, the EMC performance of equipment complying with EN 61326-1 could fall into one of six distinct categories.

Emissions

  • Class A (industrial)
  • Class B (domestic)

Immunity

  • Controlled (shielded and filtered environment)
  • Basic (domestic/commercial)
  • Industrial (heavy machinery)

On the face of it, a product tested to Class A / Controlled (poor EMC performance) can’t be distinguished from one that has passsed Class B/Industrial limits (excellent EMC performance).

What to do?

The way I suggest overcoming this and informing the end user a little more clearly about the performance of the product is to explicitly state in the DoC what levels the product was assessed against during any testing.

Example:

 

This equipment was assessed against the following Harmonised Standards:

 

– EN 61326-1:2013Electrical equipment for measurement, control and laboratory use – EMC requirements – Part 1: General requirements” (Class B emissions, Industrial Immunity)

 

I hope you enjoyed this short dive into standards land. It’s a nice place to visit but you wouldn’t want to live there!

Speak soon,

James