Category Archives: EMC Measurements

Questions on Radiated Emissions Pre-Compliance Testing

Following the recent webinar sponsored by Rohde & Schwarz, I received way too many questions to answer during the live presentation, so I’m following up gradually and posting my answers on The EMC Blog, now hosted by EDN.com.

I just posted my third article:

3. Questions on EMC pre-compliance testing for radiated emissions

I’ll be posting one more on the questions I received on general EMC topics some time in August.

In the meantime, I’ll be attending Doug Smith’s ESD/EMC three-day seminar next week and may write up a description with pictures. The week after that, I’ll be attending the annual International Symposium on EMC right here in Denver. I’ll be posting at least two articles on new products and other activities.

Your questions answered regarding cables and PC boards

Following the recent webinar sponsored by Rohde & Schwarz, I received way too many questions to answer during the live presentation, so I’m following up gradually and posting my answers on The EMC Blog, now hosted by EDN.com.

I’ve posted two articles so far:

1. Questions on cables for EMC mitigation

2. Questions on PC boards for EMC mitigation

I’ll be posting more soon on general EMC topics and pre-compliance testing for radiated emissions.

Interference from LED traffic lights and large “jumbotron”-type signs

I’m starting to receive more field reports from EMC (and other) engineers regarding the radiated emissions from LED-based traffic lights and especially from the large “jumbotron”-style LED-matrixed signs and billboards.

The LED traffic lights typically emit broadband interference, which covers the AM broadcast band within 100 feet, or so, and the larger LED signs emit harmonics well up into the UHF bands. One of my colleagues just helped resolve an issue with one of those giant signs that was interfering with an established 3G microcell in a nearby hotel. Then, just a couple days ago, I ran into a report out of Sweden regarding interference to aircraft communications from a large advertising billboard sign located near the Trollhättan-Vänersborg Airport. This was reported by the National Electrical Safety Board via their www.elsakerhetsverket.se web site.

The following is a translation of the Swedish text. While not a perfect translation, I think you’ll get the gist.

In December, the National Electrical Safety Board decision on prohibition of two billboards at Trollhättan-Vänersborg Airport, which posed a serious threat to flight safety. Measures have been taken and the interference is not currently a pressing problem.

On 16 December, the National Electrical Safety Board decision on prohibition of two hoardings sending out radio signals due to flaws in the design. The decision was made because the air traffic radio communications were disrupted during takeoff and landing. Disturbed radio communications can call from the airport or from another aircraft missed.

Troubleshooting underway

Using ban was lifted after the disturbance moved to another frequency that does not interfere with aviation radio communications. Safety Board has presented the company to correct the interference, and the provider is working to resolve the issue.

While the manufacturer claims to be troubleshooting the problem, all they did to initially resolve the interference to aircraft communications was to shift the sign’s clock frequency slightly, moving the interfering harmonics sufficiently out of the aircraft band. So, I can’t help but wonder what the harmonics are interfering with now?

Apparently, the current emission standard for lighting, IEC/EN 55015 excludes LEDs and is being revised to correct this. I guess it was thought LED lighting technology was more passive as far as interference goes. However, today’s industrial lighting designs use multiple switching power converters operating with very fast edge speeds (for efficiency) and in the 100’s of kHz, creating broadband emissions out to 200 MHz, or more. As LED lighting continues to take hold over other forms of illumination, interference reports like these are bound to proliferate. For those of you working in the lighting industry, this is a “heads up”!

EMC Integrity (Longmont, CO)

Many of you may know, as an EMC consultant, I’ve been partnering with one of the best EMC test labs in Colorado, EMC Integrity, in Longmont, north of Denver. EMC Integrity was founded by Vince Greb in 1993 and now owns two of only three 10m semi-anechoic chambers in the state (the other is owned by Hewlett-Packard in Ft. Collins). They specialize in both commercial and military EMC testing. Check them out at www.emcintegrity.com. They were recently featured in an article in the Boulder County Business Report (March 15-23, 2013). Check it out here…

I’ll be presenting a comprehensive two-day EMC design and troubleshooting short course there April 2-3 (sold out), with a possible follow-up course later this year.

EMCI_Lockhart - CastnerTechnician, Casey Lockhart running a radiated emission test at one of the 10m chambers. Photo courtesy BCBR (© Jonathan Castner).

 

A pocket-sized 5.35 GHz USB spectrum analyzer!

Always on the lookout for useful, but inexpensive test equipment, I recently ran across the Triarchy Technologies USB spectrum analyzer, model TSA5G35. The one thing that really struck me was the whole thing was built into a USB dongle, just a little larger than a memory stick. What’s more, the advertised frequency range was 1 MHz to 5.35 GHz. This, I had to see for myself.

So, is a spectrum analyzer no larger than a pack of chewing gum that you can carry in your pocket good enough for EMC analysis and troubleshooting? For a total cost of $599 (through their store on eBay), I decided to take a chance and run this remarkable PC-based analyzer through the ringer.

IMG_1482Figure 1 – Photo showing the analyzer with supplied USB extension cable and 30 dB attenuator.

16 MHz Osc with Mkr-600Figure 2 – A screen capture of a series of 16 MHz oscillator harmonics.

SPECIFICATIONS – Basic specifications include frequency coverage of 1 MHz to 5.35 GHz, resolution bandwidths of 50 through 500 kHz (not selectable), frequency spans from 1 MHz to 1 GHz, input level range of -110 to +30 dBm (using the supplied 30 dB attenuator for the higher power levels), and typical noise levels of -80 to -100 dBm (depending on the span and RBW). The maximum power level is +20 dBm for 1 minute (without the external attenuator) and +/- 25 VDC, which is excellent protection for this little instrument. The reference level range is -60 dBm to 0 dBm (no external attenuator) or -30 dBm to +30 dBm (with the external attenuator. the usable display range is 80 dB with a noise floor of -115 dBm at a 5 MHz span and -60 dBm reference level at 1 GHz. Amplitude accuracy is specified at less than 3 dB. All in all, not to bad for this little guy.

For more of this hands-on review on Test & Measurement World, click here…

Unusual EMI Sources

These unusual EMI sources may be used to perform pre-compliance testing (radiated or conducted immunity) to help determine the immunity characteristics of your circuits or product.

1. Chattering Relay (120VAC powered) – can produce strong broadband emissions all the way out to at least 1 GHz.

IMG_1423

2. 3 VDC Motor – produces strong emissions out to about 750 MHz.

3V Motor 1

3. Pocket Plasma – produces broadband frequencies up to 10 MHz.

IMG_1419

Click here for more details…

Question on Current Probe Calibrations

I recently received the following question on how to calibrate current probes and thought you’d be interested.

Question: Good morning.  I read your article, “HF Current Probe:  Theory and Application”, but now I have a question I’m hoping you can help me answer.  I am attempting to measure the transfer impedance of a current monitor probe using a probe calibration fixture or jig.  To keep the setup simple, I am using a signal generator and a power meter.  As in your example, I am setting the generator to source 0dBm and I will verify it with the power meter and sensor through an adapter; I will then connect the generator’s output and current probe to the calibration fixture and measure the probe’s output using the same meter and sensor.  This all works fine until the input SWR of the calibration jig reaches about 1.3 at 100MHz.  From that point up to 400MHz, the SWR of the jig reaches 3.4.  It appears that one would be measuring both the current probe’s insertion loss and the calibration jig’s mismatch loss.  Would it then be best to establish the reference by measuring the output of the signal generator while it is connected to the calibration fixture (without the probe inserted), so as to include the jig’s mismatch loss in both the reference and measurement sweeps?

Answer: You’re on the right track. You need to normalize out the effect of any mismatch from the jig setup. There are several methods for calibrating current probes. If you have the jig, that’s great. Basically, what you’re trying to do is measure the current accurately versus frequency – a not so trivial task to keep the current fixed as frequency changes. The problem is that any parasitics (R, C, L) in the wire to be measured can greatly influence the current value. That was the problem I was running into when measuring the wire in the referenced article. I tried to keep the value of current fixed by inserting a small resistor in series and measuring the voltage drop, keeping the this voltage drop steady by adjusting the RF generator output. It’s much better to use the 50-Ohm jig, but there will still be mismatch errors, which may be somewhat alleviated through the use of 6 to 10 dB attenuators. The goal is to measure the current through the probe versus the voltage at the probe terminals. Dividing the terminal voltage by the current gives you the transfer impedance. I’ve attached a few references.

Here’s a recent article from Interference Technology.

Teseq also has a calibration procedure within the instructions for their test jig, and look under the “downloads” tab.

Dr. David Pommerenke, of Missouri University of Science and Technology (MST), authored a paper with Ram Chundru and Sunitha Chandra on “A New Test Setup and Method for the Calibration of Current Clamps“, which runs through the historical calibration methods and then suggests an improved method.