D. C. Smith Consultants
High Frequency Measurement Website
Douglas C. Smith
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HF News: Analog RFI, ESD, and More
Issue 4
March 13, 2017
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Overview: Ferrite used in ESD/EFT/EMC mitigation is the subject of the week. As usual there are lots of engineering links this week, some funny and some informative.
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General News: Interference from pulsed sources of noise like ESD and EFT (electrical fast transient) continue to become more of a problem these days and much of my recent work has been to help clients overcome problems resulting from pulsed EMI. Although there are some general rules, each case is different and often requires a custom approach to the problem. Most designs with problems could use multiple changes to the designs to make the designs good, but only one change is usually required. The catch is to find just what is necessary. Often fixing all the design problems are not necessary to get a product to work as expected.
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What I Have Been Doing: I have been pretty much consumed by the preparations and presenting at EMC Week in Boulder City, NV. In addition, a few clients have been soaking up time so I was unable to get HF News out the last two weeks. But I am back on track now and this week I want to discuss the use of ferrite cores.
For many years I have known that two turns of a cable through a ferrite core works better below 200 MHz and a single pass of a cable is best for frequencies above 200 MHz. I was browsing the Fair-Rite website recently and they have a great impedance plot that shows this convincingly. Figure 1 below shows the plot:
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Figure 1: Ferrite Impedance Plots
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The upper plot shows the impedance inserted into a cable in common mode for one pass of the cable through the ferrite. Notice that the impedance is mostly resistive. This is desirable as adding inductance will just tune the cable resonance to a different frequency whereas a lossy core with a mostly resistive impedance will damp resonances.
The lower plot is the most interesting of the two. Notice that below about 200 MHz adding a second or third turn of the cable increases the series impedance injected by the core on the cable whereas above 200 MHz additional turns actually reduces the added series impedance of the core. As the frequency approaches 1 GHz, the core itself is not performing well and we need a different core material.
Here are a few more links from Fair-Rite on engineering kits, always useful to have around the lab:
Type 43 cable kits
Type 31 cable kits
Listing of engineering kits
If you have links to more ferrite kits or information, let me know and I will publish them in the next issue.
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Upcoming Events:
Let me know if you would like to post your event here.
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Feel free to contact me anytime!
And now a word from our sponsor, me that is! If you are interested in finding hidden flaws in your design that may become lab or field disasters, or quickly determine the cause of a field problem you already have, I have developed tools that accomplish these tasks quickly and easily that standard engineering analysis cannot. Many of these tools and techniques are not available in the literature. Most design problems I work on are understood, and even fixed, in a few days, even after weeks or months of effort by in-house resources have failed. Call me to discuss.
Douglas C. Smith
PO Box 60941
Boulder City, NV 89006
Email: doug@dsmith.org
Tel: 702-570-6108
Cell: 408-858-4528
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