Tag Archives: Fields

Why Electronic Products Produce EMI Part 2

In the Blog entitled Why Electronic Designs Produce EMI Part 1, we discussed why radiated emissions occur in electronic designs.  A simple way to visualize electronic signal current forward and return paths was provided, and mismatched signal path impedance was briefly mentioned as one reason for signals to radiate. In this article, the significance of signal path impedance is described in greater detail.

We can start by asking the question, what kind of signal path will not radiate? Mathematics aside, the simple answer is that a signal path that matches source, path, and destination impedances will not radiate. Here is a simple illustration of a matched impedance signal path:

matched impedances

In the above illustration, if Zin=Zpath=Zout across the entire spectrum of frequencies from DC to light and beyond, almost any signal which is sourced from the buffer will not radiate. Why not? Because all of the energy that is sourced from the buffer will be delivered to the load. What happens if there is a mismatch along the way? The simple answer is that mismatched impedances cause signals to reflect, which equates to ringing, which results in radiated noise. This is as true for high speed communications signals as it is for power supply outputs.

How much ringing will there be and at what frequencies? This depends on how much impedance mismatch occurs in the path, how much energy is being transferred, and what the resonant length of the path is.

In both high speed data and power supply applications, circuits are generally designed to have very low impedance at the source drivers, and very high impedance at the destination loads. Also in both high speed data and in power supplies, very sharp edges on switched signals are considered desirable because sharp edges with a lot of energy result in fast data transfer and maximum efficiency switching supplies. Unfortunately, these two conditions also create an ideal environment for signal reflections, especially at high frequencies.

Why do mismatched impedances reflect? Imagine a floodgate opening, and downstream a huge rock blocking the river. What will happen when the current reaches the rock? Water will splash, of course, it will bounce back or reflect into the oncoming stream of water. Water might even splash beyond the edges of the river.

Why do sharp edges result in reflections? The spectral content of a square wave contains odd harmonic signals that can go to very high frequencies. This is why processors operating with a 10 MHz clock or switching power supplies with a 400 kHz clock can produce 250 MHz noise, because of the amount of energy used in switching creates fast rising and falling edged signals. Even a DC switch can produce momentary reflections up to very high frequencies.

So, what does this all have to do with blocked or open return paths described in the previous blog? Even reflected energy seeks the path of least resistance, whether it is reflecting towards the source or the load. If the path of least resistance is a radiated path back to the source supply rather than the circuit board or cable signal / ground path, the reflected energy will radiate.

At Sunrise Electronic Innovations we have decades of experience in designing emissions compliant electronic products. We help customers to reduce emissions on existing products and we provide design and layout review and educational services to improve and cost reduce products. To gain the benefit of our experience, give us a call at 678-358-8775.

 

 

Why Electronic Products Produce EMI Part 1

Electronic designers may avoid considering electromagnetic compatibility because their exposure to electromagnetic field theory was so mathematically intensive. While mathematical models are important, focusing on the math can distract a person from appreciating the practical aspects of how electromagnetic fields behave.

In my experience, the easiest way to understand EMI is to think of water. Water seeks the path of least resistance, and water wants to return to its source.

In EM terms, what is a source? Any place where current originates. A buffer, a power supply, a driver, or even a bypass capacitor, an inductor, or ferrite bead can be thought of as a current source.

Unlike water, when electrical current flows, it generates an electromagnetic field. And the EM field that current generates in turn influences the path along which the current flows.

Ideally, in electronic circuits, all return currents follow paths back to their sources that match the paths that the currents originate from. Consider the diagram below:

figure1Figure 1: Optimal Current Paths

The arrows indicate both primary current flow paths and return current flow paths through ground. When return current flows along a path that matches the source path, the EM fields generated by the current flow cancel each other, and the circuit does not create Electromagnetic interference. Another way to look at it is that all the current energy stays within the intended pathways.

Like water, electric current energy follows the most efficient path back to its source. If the intended pathway is not the most efficient route for return current to follow, the electric fields generated by the forward and return currents will choose a more efficient path, and emissions will result.

Now consider the figure below, which illustrates a problematic return path in the buffer circuit described above:

figure2

Figure 2: Problematic Current Paths

Notice in figure 2 how the most efficient return path for the current being sourced by the buffer is no longer the short path back to the buffer, but rather a long looped path through earth or chassis ground and the mains power source.  When tested for emissions, the above illustration may produce either conducted or radiated emissions, depending on the frequency characteristics of the buffer sourced current.

Why would electrical energy choose a long path rather than the intended short path? This is where a little math may be required, or at least an understanding of what the math predicts. One reason might be that the longer path happens to match a resonant wavelength of some harmonic frequency of the alternating buffer current. Another reason might be that the intended return path presents a high impedance at the offending frequency and actually blocks the return current. Mismatch between the buffer circuit source, path, and destination impedances can be a factor as well, causing undesirable reflections in the buffer sourced current.

Whatever the cause for the undesirable emissions, understanding the behaviors of electromagnetic fields can help a designer to “design out” potential EMC issues prior to testing. Looking at a design as a series of current sources that require adequate return paths can assist in this understanding.

Part of our job at Sunrise Electronic Innovations includes educating our customers while doing design reviews and product pre-screen testing. As we educate our customers, their designs improve to the point that little to no modification is required for passing certification testing. To gain the benefit of our experience, please give us a call at 678-358-8775.