Bonding the shield well to the shielded enclosure or adding on-board common mode chokes or installing an external common mode choke around the product end of the I/O cable are typical solutions. These higher frequency antennas are available from Q: What can you do if you notice the emissions noise is actually on the shield of the cable?Ī: That’s typically where you find these noise currents (also known as common mode currents) traveling along the outside of your shield. For very high (above 6 GHz) emissions, I use the smaller PC board log-periodic antennas and higher frequency broadband preamplifiers. Basically, I use the same tools and troubleshooting techniques. Q: what do you troubleshoot frequencies above 1GHz?Ī: Assuming you’re asking what tools and techniques I use for troubleshooting radiated emissions above 1 GHz. This should help you determine a “safe” margin when using your 3m chamber and may even provide the justification required by other country regulatory agencies. This will provide you a sort of calibration curve between the two measurement distances versus frequency. One thing you might consider is to take a comb generator with some standardized (electrically short) antenna and compare between your 3m chamber and a commercial 10m chamber. In other words, the assumption of “plane waves” is not quite true and so the distance factor from the product under test will also depend slightly on factors of 1/r^2 and 1/r^3 terms, rather than the simple 1/r term for plane waves. Finally, depending on the definition you use (typically 1/6 th wavelength), you’re approaching the near field at the lower frequencies. Also, the 4m antenna height can’t normally be achieved in a 3m chamber, so you’re not fully comparing the higher “take-off angles” of fields emanating from the product under test between the two measurement distances. I always suggest that margins from the limit be increased when using 3m chambers, if at all possible. The issue for 3m arises mainly at the lower frequencies (say, under 200 MHz) where the chances of reflection errors might be greater. How to justify this? Should final reports always be at a 10 m test site, not a 3 m chamber?Ī: While a 10m distance seems to have been adopted by the standards committees as a “standard”, not everyone can afford a 10m chamber. We are questioned by Japan as to why we do not use a 10 m chamber, since limits are in terms of 10 m in all of the standards, not 3m. Q: We typically use a 3m chamber to test our IVD instruments, as seems typical in the US and Europe. However, I’ve seen cases for radiated (and especially ESD and other impulse type signals) that will reset the microprocessor IC or cause glitching in digital signals. Q: For immunity, I thought an analog circuit was more susceptible than a digital circuit?Ī: Generally, you’re correct. Of course, the objective is to eliminate the common mode currents on all your cables so cable emissions don’t enter in to the equation! It really depends on how tight the folding is. Folding (serpentining) the cables tends to cancel the field in that section, but may produce a minor amount of vertical emission. If they droop over the edge of the table, they’ll generally produce a combination of horizontal and vertical emissions. If the cables are oriented horizontally, they’ll generally product horizontal polarized emissions if vertical, then vertical emissions. I’m assuming these are all cables attached to the product and spread out around the measurement table. For radiated immunity, I feed a small RF field into one of these small loops and run the field around the PC board, looking for areas of sensitivity.Ī: Good question. You’ll likely require a broadband preamplifier to boost the very low signals from the smaller probes. You can also make your own by fashioning a small loop at the end of a piece of small-size coaxial cable. Beehive Electronics, Aaronia, Langer and Rhode & Schwarz all make near field probes with very small loops that will yield good resolution. However, for identifying emission sources, I’d use a very small loop probe. For emissions, I’d test like any other board. How do you handle first EMC measurements for very small PC boards (like the size of a pencil)?Ī: I did have to evaluate a medical product with a very small PC board. Q: What you describe is good for big PC boards. Several questions on EMC measurements and product design were asked during my recent webinar hosted by Interference Technology Magazine and sponsored by Rohde & Schwarz.
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