EMC Basics: Losses with EMC

Welcome to part three of our five-blog series on EMC basics!

In the last two posts, we went through an overview of EMC and what it has to do with chip bead ferrites, including how to read a datasheet.

In this post, we will explain losses with EMC — how to identify them, how to control them, and how to calculate insertion loss in your design.

EMC Losses: New Prime vs. New Double Prime

The reality of life and electronics is that everything has losses. The key is to understand what your losses are and how you can control them as best as possible.

Every core manufacturer specifies new prime and new double prime. New prime is the inductive reactance in this frequency range where it is best used as an inductor. In this image, new prime is represented by the blue line.

New double prime is the resistivity where the part is best used as a filter (represented by the pink line). The same component can be used for either filtering or inductance, depending on the frequency that it is used. Refer back to our discussion on the crossover frequency, and you will be able to identify these frequencies.

The Best Materials for Different Frequencies

Different materials have different frequency responses. Here, we're providing you with a guideline for what materials are best suited for what frequencies. Keep in mind that this is just a guideline based on a typical situation. As with anything else, there are always exceptions.

Iron powder, which is the green portion of this graph, is typically used for lower frequencies for noise attenuation because it has the highest core losses at frequencies between 200kHz to 4MHz.

Manganese zinc, the red portion of the graph, is typically used for frequencies between 3MHz and 60MHz.

Finally, nickel zinc, the purple portion of the graph, is typically used at frequencies between 20MHz to 2GHz. This is because it's very resistive at higher frequencies.

Insertion Loss Calculation

If you enjoy performing long mathematical equations, you’ll love this next part.

If you don't, you may enjoy it even more. That’s because we have some tricks and tips to help you save both time and energy spent on mathematical equations and using some of our models.

As you can see, there are a lot of difficult calculations that determine how much impedance you need for a component to be within a specific certification for attenuation. That is represented by the equation here for the system attenuation (A) and the impedance (ZF).

Because of the parasitic properties of the component, it becomes very difficult and time-consuming to get the mathematics right.

What Type of Filter to Choose

This picture shows a simplified representation of your design. You have a source and a sink, and the interaction of the source and the sink will cause some sort of noise.

Depending on the amount of noise created and at what frequency, that will influence what type of filter you need.

Assumed Practical System Impedance

If you have time and you're up to the mathematical challenge, feel free to use the equations from the image above. If you want an easier way, you can use this nomogram.

What you see in the legend here are some typical applications: ground planes, supply voltage lines, data signal lines/clock/video signal/USB, and long data signal cables. The assumed practical system impedance on the left gives you an idea of the impedance of the entire system for that application.

Remember that these system impedance values are approximations, not exact representations for everything. But as you will see, for our purposes, the approximations work just fine and actually save us a lot of time. And most importantly, they save us from having to do those painful mathematical equations!

What Type of Application to Use

So how do you know what type of application to use from this example?

Well, you know your board. You can see the ground plane, the supply voltage line, and the data signal line.

If it's not as obvious from the bottom of the board, you can see the top of the board as well. A quick glance will show you the connectors, the power plug, and the power lines.

Calculating the Filter Impedance

For this example, we need to attenuate 12dB at 125MHz. We know the application is a power cable, and we have 10 ohms of impedance.

You can see in the equation for filtering (ZF) that we can plug in these values, and the result is a filter impedance of 59.6 ohms. Then, if you are feeling up to it, you can double-check your work with the equation for attenuation (A). The result is 11.99dB.

Using a Nomogram

Let's apply the same scenario, but this time let's use a nomogram. You’ll see that we quickly come up with the same conclusion without doing any complex mathematical equations.

Keep in mind that the frequency is built into the impedance. Therefore, you don't have to use the frequency when using a nomogram. You know you need 12dBs of attenuation (just like before), you know it's a power cable, and you know it needs an impedance of 10 ohms.

Simply follow the black solid line for supply voltage applications until it reaches 12dB. There, you find your results of 60 ohms impedance — the requirement we already determined.

So the answer came out the same both for the long mathematical calculations and the nomogram. Remember that nomograms are provided in our catalog for your convenience.

And now you know how to easily calculate insertion losses in EMC!

In the next post in our blog series on EMC basics, we’ll cover common mode vs. differential mode noise and how to combat them. Stay tuned!