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Q&A: More EMC issues

30 Dec 2013  | Kenneth Wyatt

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Q: what do you troubleshoot frequencies above 1GHz?

A: Assuming you're asking what tools and techniques I use for troubleshooting radiated emissions above 1GHz. Basically, I use the same tools and troubleshooting techniques. For very high (above 6GHz) emissions, I use the smaller PC board log-periodic antennas and higher frequency broadband preamplifiers. These higher frequency antennas are available from www.ka5vjb.com.


Q: What can you do if you notice the emissions noise is actually on the shield of the cable?

A: That's typically where you find these noise currents (also known as common mode currents) traveling along the outside of your shield. 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. Even better is to design your PC board and system designs properly to avoid or minimise these common mode noise sources in the first place.


Q: How do you validate the E-Field power level when using test antennas in a test bench area?

Q: For pre-compliance testing, how do you correlate your measured signal to compliance levels with homemade antennas?

A: When testing with a sense antenna 1m away from the product under test, it's possible to get "quick and dirty" answers by adjusting the limits upward by 20 dB from the 10m limits. Understand, this is only a very rough ballpark limit, as you'll be well into the near field for the lower frequencies. See the discussion above relating to 3m versus 10m testing for more detail on some other issues. It's much safer to set up a calibrated EMI antenna 3m from the product and use 3m limits for more accurate results. The advantage of setting up an antenna close to the product is that you can get immediate results (did the harmonic go up, down or no change?) during the series of experiments and potential fixes. This technique will really boost your efficiency. It's also very helpful to have calibrated test data in hand showing where harmonics are close to (or over) the limit. Realise, though, that a 10 dB decrease at 1m does not necessarily equate to a 10 dB decrease at 3m or 10m. However, any decrease will be a good thing. You'll just need to optimise the emissions as best you can on the bench and then have the product retested in a decent chamber on occasion in order to check progress.


Q: When do we use an E-field versus a B-field probe for EMC testing?

A: H-field (or B-field) probes are best used to detect H-fields – typically from wires, cables or traces where you might have a high di/dt (currents in loops). E-field probes are best used to detect the E-fields from enclosure slots or gaps. They are also useful for detecting high dv/dt, such as in switching power supplies. Understand, though, that when measuring in the near field the electromagnetic wave will have components of both E- and H-fields. It mainly depends on which field might be dominant (currents or voltages).


Q: I am confused about the purpose of the pigtail. What is the purpose or the point and why is it useful?

Q: Can you elaborate on the pigtails please? I didn't understand that portion of the presentation.

Q: What's the recommended method in terminating cable shields?

A: First, let me state that "pigtail connections are bad news" and can generate high levels of common mode currents, which tend to radiate and cause your product to fail. The use of pigtail connections is a "cheap and dirty" way to connect cable shields to their connector shield terminations. More expensive connectors have provisions for connecting or clamping the cable shield in a 360-degree bond, which is ideal. The longer the pigtail used, the worse you can expect emissions. If you're forced to use poorly designed connectors, it's recommended to use two (or more) short pigtails to the connector shield, one on each side of (or surrounding) the internal wires. This will tend to cancel the resulting fields.


Q: I have some really sensitive pressure sensors and are picking a lot of noise from radiated frequencies on the immunity tests. I can see the 2Hz frequency on the product, as if it were the signal I am trying to measure! Even when shielding them with EMI plastic and/or metallic shields. These shields are not connected to ground nor earth, because doing that worsen the results. Maybe I am doing something wrong?

A: If your sensor is designed to pick up 2Hz signals, it should be relatively easy to add simple low-pass filtering at the input in order to remove higher-frequency noise. You'll want to ensure the sensor electronics is completely shielded for additional protection. Be sure the low-pass filter is located right at the sensor input and just inside (or at) the shield boundary.


Q: With high frequency these days how can you have ventilation holes in your shield at 1/20 wavelength?

A: The 1/20th wavelength of a slot or seam is merely a rule of thumb, which approximates about 20 dB of shielding effectiveness at that frequency. When designing ventilation holes or slots, you need to consider the highest frequency used (or harmonic produced) by your product. Then take 1/20th wavelength at that highest frequency and calculate the maximum hole or slot size from that. For example, for 1GHz, 1/20th wavelength is about ½ inch. If you need to go higher in frequency, then it's generally best to use patterns of small round holes – maybe ¼ inch, or smaller, in diameter.

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