The shopping list for DC Conducted Emissions pre-compliance testing

by Ignacio de Mendizabal, 14/8/2024

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Hello Reader,

Happy to have you here

Today, we will prepare everything we need to measure DC conducted emissions using Line Impedance Stabilization Networks (LISN). We will use two of the most affordable models and you will eave this chapter with a shopping list with all the components.

Standards

There are many different standards defining the test set-ups, limits and frequency bands. For industrial products, the applicable standards will be the IEC-61000-6-4. You can get the official documents in the IEC website or, as an alternative, and much cheaper and still legal, in the Estonian Centre for Standardisation website.

The standards define the official test set-up, they describe what you will find in a certification tests. Since we are not building a certificated laboratory (yet?), we will adapt it to have accurate measurements without spending all our savings in a new laboratory set-up.

Line Impedance Stabilization Network (LISN)

LISNs filter the noise from the power source and guarantee a constant impedance within the measurement range. They have a 50 ohm output to measure the emissions coming from an electronic system. In this experiment, we will analyze these two:

• ​Elektor dual DC LISN (150 KHz - 200 MHz)​
• ​Tekbox ТВОН01 (100 KHz - 110 MHz)​

Elektor dual DC LISN

It comes unassembled, so I had fun assembling the different parts within the case. It is very solid and it is clear that the PCB has been carefully designed to manage hundreds of MHz. As an open project, the source files can be downloaded (you will need to create an account)

• ​Dual DC LISN Schematic file​
• ​Dual DC LISN PCB file​

I contacted the manufacturer to get the impedance curves. I will share them in the future once I get them.

The most interesting feature of this LISN and that I look forward to testing, is its ability to measure common-mode noise and differential-mode noise. It can make the troubleshooting fast and smooth the design of EMC filters, avoiding trial and errors and over engineering.

Tekbox TBOH01

The Tekbox LISN comes ready to use and the design files can be downloaded with the following link:

• ​TBOH1 BOM, layout and schematic files​

The LISN impedance curves reflect what the Device Under Test (DUT) will see, a constant impedance within the frequency range. The curves look for the o be sure that the LISN is good, we check the LISN impedance curves.

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We see that:

• The impedance is constant and within the limits defined by the CISPR standards BUT
• It is not constant, but controlled. It is not enough to assume that it is a 50 flat curve. We will need to add correction factors in our measurements to get accurate measurements.

Both LISNs have an impedance of 5µH//50Ω and are rated for 60 V and 10 A. The different features are the frequency range, channels and the attenuation.

For industrial measurements, we normally focus on the band between 150 KHz (sometimes 9 KHz) up to 30 MHz, so the impossibility of measuring hundreds of MHz is not really important.

A capacitor

It sounds simple, right? It is. We need a capacitor. And why is it not included in the LISN? Because its value depends on the standard. For the CISPR25, this capacitor is 1 uF, while for the DO-160 (avionics applications), this capacitor is 10 uF. The LISN schematic shows it:

A metallic surface

To give return path to the high-frequency fields and bond the LISNs, we need a reference plane. It needs to have a very (very) low impedance. In the official tests, the material is copper, which can be expensive and difficult to find. Our goal with pre-compliance set-ups is to build good-enough conditions to reduce the risk of EMC problems at later stages.

One option, mainly when you have limited space as I do, is the Tekbox TBGP Rolling Up Ground Plane​

Attenuator

An extra piece that is nice to have, even not compulsory is an attenuator or, more specifically, a transient limiter.

Why?

Because we are humans, and we forget things, mainly when we are busy and concentrated looking for a solution to a frustrating problem.

When a voltage that switches fast (the supply voltage) is applied on a capacitor (the one we described above and others present in the LISN), a fast current pulse is generated. This pulse can be high enough to damage the input of your Spectrum Analyzer. And that is something that we want to avoid at any cost, right?

Another reason to have it is to add some extra attenuation, so we do not overload the input ADC.

The Elektor Dual DC LISN has an internal attenuation of 10 dB, but not the Tekbox one.

A good option is the Tekbox TBFL1. There are other options such as the R&S ESH3-Z2 or the Com-Power LIT-930A, but their price is at least twice of the one from Tekbox.

EMI Receiver or Spectrum analyzer

We need a Spectrum Analyzer or an EMI receiver, so we can make Quasi-Peak and Average Measurements. I will use the SVA1015X, with a frequency range of 9 kHz to 1.5 GHz. The only thing I will need on the top of it are some RF adaptors: SMA-TNC for the Elektor dual LISN and BNC-TNC for the Tekbox one. Here there are different combinations possible, depending on the cables you have. Then, I added to the list the adaptors that are useful for me. Check which ones are good for you.

Advancing future steps, there are a few things we should remind:

• We need to consider correction factors of the LISNs, so we compensate the impedance variation.
• If we want to replicate the CISPR measurements style, we will need a specific Software to scan the whole frequency range.

Today was just the shopping list, we will go through them in future chapters.

Shopping list

Putting all together, here is what you need to buy to start measuring conducted emissions. Please, do run and buy everything at one, you might not need every single item of the list.

• ​Standards - 24.40 €​
• ​Elektor Dual DC LISN - 139.95 €​
• ​Tekbox DC LISN - 274 €​
• ​A capacitor - 0.2 €​
• ​Reference plane - 320 €​
• ​Transient limiter - 318 €​
• ​Spectrum Analyzer - 1359 €​
• RF Adaptor - 50 € - TNC/BNC and SMA/TNC​

Read more

I recently added to the website a collection of articles.

• ​Understanding EMC - Conducted EMC​
• ​Design and Simulating EMC filters with LTspice​
• ​A brief review of the tools and parameters of EMC Testing​

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You can find me on Linkedin and on the Website​

Best,

Ignacio