Transmission line losses—driven by skin effect and dielectric properties—play a critical role in degrading high-speed signal integrity and eye performance.
Transmission line loss directly affects eye diagram quality, with around −12 dB at Nyquist marking the limit before signal integrity rapidly degrades without equalization.
When we are testing a product on our bench top for EMC, we are in close vicinity to that product in a typically noisy environment. All we can measure is the near field, the electric or magnetic field strength in close proximity to our product. It’s important to keep in mind that near field measurements are not the same as the far field (3 m or more) measurements in the FCC Part 15 Radiated Emissions test described earlier, and here's why.
When we have some source that's generating radiated emissions, depending on the current distribution and the shape of that source, we will have all sorts of multipole moments radiating: dipoles, quadrupoles, hexapoles, etc. There will be different shapes of electric field radiating based on the pattern of loops that the currents make. They all radiate, but the radiation field from the different multipoles drops off with distance at different rates. It's the dipole field that keeps on going and contributes to the far field radiation field. And so unfortunately, when we’re close to the product, we’re measuring all the near field strengths, which we’ll rarely see in the far field because they drop off so quickly.
That means that we have to be very careful interpreting the results from near field measurements. The near field is going to give us the radiation from all the moments, not just the dipole terms. The downside is that what is measured in the near field is not always a representation of what will be measured in the far field. A lot of near field radiation may not be indicative of FCC failure. On the other hand, if we fail an FCC test, then we absolutely will see near field emissions, so anything that can be done to reduce the near field emissions will help with passing an FCC far field test, although it is no guarantee.
Eric Bogatin presented a webinar Pre-Compliance EMC Testing with a Real Time Scope. That webinar covers more details on this topic and includes demonstrations of tests for detecting radiated emissions. Click the button below to watch the webinar, or other related webinars available from that same link. Or download the transcript to preview the webinar content.
Eric is a professor in the EE department at Colorado University, Boulder and a Fellow at Teledyne LeCroy. He has a BS in Physics from MIT and a PhD in Physics from the University of Arizona, Tucson. He enjoys DesignCon and writing for SI Journal.
Transmission line losses—driven by skin effect and dielectric properties—play a critical role in degrading high-speed signal integrity and eye performance.
Transmission line loss directly affects eye diagram quality, with around −12 dB at Nyquist marking the limit before signal integrity rapidly degrades without equalization.
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