Monthly Archives: February 2012

Matched Set of Current Probes Arrived!

Posted on February 23, 2012 | Leave a comment

I just received my matched set of Fischer Custom Communications (FCC) model F-33-1 current probes! Yipee!

This is actually a pretty big deal, as matched current probes will allow me to make some very unique measurements during troubleshooting. Many of these techniques were pioneered by fellow colleagues Michael Mardiguian (EMI Troubleshooting Techniques) and Doug Smith (High Frequency Measurements and Noise in Electronic Circuits).

I’ll be showing you some of these techniques in future installments.

Fischer F-33-1 current probes (matched set).

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Troubleshooting Radiated Emissions at your Bench – Part 1

Posted on February 23, 2012 | 2 comments

What do you do after returning to your workbench with a product that has just failed radiated emissions? In this multi-part series, I’ll describe simple and low-cost ways I use to help my own clients solve these issues. Most of the time, it’s possible to set up a simple 1 to 3 meter “measurement range” and determine whether a potential fix is required, or not.

PHILOSOPHY

First, a little troubleshooting philosophy. In many cases, you’ll run into more than one emission source causing the same harmonic frequency. The result is that you might apply a fix and the harmonic will do three things – either get reduced, have no change or better yet…get larger! It won’t be until you apply fixes to ALL the sources that you’ll yield positive results. This is what makes chasing down emission problems such a joy(!), I mean “challenge”!

There is also the issue of “balloon effect”; that is, you’ll beat down one frequency, only to have one, or more, pop up higher! It’s like squeezing a balloon in the middle – both ends get bigger! Often times, this is the result of resonances within your cabling or on the PC board.

In this installment, I’ll describe some simple reference antennas I use (you’ll be surprised) as well as setting up an area on your workbench where you can troubleshoot and apply potential fixes and really see whether you’re making progress, or not.

ANTENNAS 

The antenna you select should ideally be somewhere near resonance for the frequencies of concern, however, it’s not really that critical for troubleshooting purposes. So long as the antenna is fixed in length and fixed in place on the bench, you’ll receive consistent results. During troubleshooting, it’s more important to know whether the fix is “better” or “worse” or “no change” and as long as the test setup doesn’t change, the results should be believable.

Low-cost EMC antennas I use for troubleshooting, based on television "rabbit ears" and a UHF folded dipole.

Now, EMC antennas are not inexpensive, as you might imagine, so for general troubleshooting, I tend to use a couple television antennas – a pair of “rabbit ears” and a UHF “bowtie” (with TV balun to match 50-ohm coax). If the workbench is wooden, I’ll extend the antenna to approximate resonance (if possible) and tape it down to the bench with duct tape. If the bench is metallic, I’ll find a non-conductive support and position it some distance away from (or above) the bench. I usually use a test distance of about a meter, but as long as you can see the product’s harmonics on a spectrum analyzer, you’ll be able to determine your progress. Sometimes I need to insert a low-noise wide-band preamp between antenna and analyzer.

Now, obviously, ambient signals from broadcast radio, television mobile phones and two-way radio services will tend to interfere with observing the product harmonics. You may need to bring the antenna closer or set up the troubleshooting measurement in a basement or building interior away from outside windows. I usually record the known harmonics of concern and try to characterize them in relation to other nearby ambients.

CURRENT PROBES

If I know that one, or more, cables are the dominant radiation source, I might use a current probe to monitor the common-mode currents flowing on the cable, rather than an antenna. This also helps reduce the ambient signals, because current probes are generally shielded against e-fields and tend to be poor antennas. I’ll attach the probe to the cable with dominant emissions, moving it back and forth along the cable to achieve a maximum, and then fix it in place.

Commercial current probes can measure rf currents flowing on I/O cables - a very typical issue for radiated emissions issues.

BENCH

Next, I’ll clear off the workbench and (assuming the product is small) find a convenient place for it where I can work on it without moving it around much. I also place reference marks on the bench with tape, so I can reposition it for repeatable measurements. At that point, I’m ready to begin the troubleshooting and fixes while watching the emission levels.

Now don’t make the mistake of assuming that a 10 dB reduction on the bench with a one-meter test distance translates to the same reduction when measured at the test facility at a ten-meter test distance! During troubleshooting, we’re likely working in the “near field” where test distance is determined by terms of 1/r squared and 1/r cubed. At ten meters, we’re likely in the “far field” (plane waves) and the distance factor is closer to 1/r. Where “r” is the test distance. You can be fairly confident, though, that a reduction on the bench will equate to some reduction at the site.

More in the next posting!

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Case Study – Radiated Emissions from LCD Displays

Posted on February 22, 2012 | 8 comments

I recently had a chance to troubleshoot a controller display with very high emissions in the 90 to 300 MHz region. This was a product inherited from an acquisition, so they had to fix the design after the fact – not an uncommon situation.

I’ve dealt with the issue of radiating LCD displays before, so had some ideas on an approach already. LCD displays use a high-speed “dot clock” signal to drive the individual pixel elements. This is frequently in the order of a few hundred MHz and is routed via a ribbon cable from the clock driver and processor circuitry. Unfortunately, and for reasons beyond me, most LCD display modules are generally comprised of a sandwich of two floating pieces of aluminum – a front bezel and rear shield and mounting plate. These plates often have a gap between them as well as sometimes lacking a bonding connection back to the chassis or product enclosure. This results in “floating pieces of metal”, which couple energy and radiate. On top of that, the OEM display assembly was packaged in a plastic enclosure, which was inserted into a large rectangular hole in the metallic controller housing and clamped from the rear. In other words, we have this large radiating assembly with no easy way to bond it to the primary metal controller housing!

I first probed around the whole assembly to confirm the ribbon cable and front bezel were the dominant radiators. I used the Beehive Electronics h-field probes connected directly to the handheld TTi PSA2701T spectrum analyzer. This took just a few minutes and proved the cable was the “hottest”, with large emissions also radiating out the front of the display.

Closeup showing the ferrite choke on the dot-clock cable.

I treated the dot-clock cable first, because that was easiest. Unplugging the cable from the PC board, I slipped a small flat ferrite choke (Laird 28R0898-100) over the cable and reconnected it. This immediately reduced the emissions as much as 12 dB. The was no effect on the display quality.

The lower frequencies, while improved, were still fairly high, so the next step was to disassemble the display getting right down to the LCD module itself. After confirming the front bezel and rear shield were indeed floating, I used copper tape to temporarily bond the two together. I then confirmed the LCD module was supported by plastic pins, so was not bonded to the rest of the display assembly. I took additional copper tape and ran it between the rear shield of the LVD module and steel sub-chassis of the display. I also ran several pieces out from the sub-chassis to the inside of the metal enclosure. The result was that all metal pieces were now bonded together. After connecting the tape to the enclosure, the emissions had been reduced from 9 to 25 dB! We later “cost-reduced” by removing most of the unnecessary fixes. After a few experiments, we were able to remove all the copper bonding between the display assembly and controller housing, but the two nearest the source. After eight hours work…a compliant product!

Closeup of the corner of the LCD module, showing the gap around the edge of the module. Copper tape was temporarily applied as a troubleshooting fix.

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Low-Cost Handheld Spectrum Analyzer

Posted on February 22, 2012 | 1 comment

The Thurlby Thander Instruments model PSA2701T. Photo courtesy TTi.

One specialty I offer my clients is troubleshooting and determining potential fixes for their products in order to get them to comply with worldwide EMC standards. Ideally, we want to do this prior to going out for compliance verification testing. As I travel a lot in my job, I like to take the minimum amount of test equipment possible. One of the fundamental pieces of gear is the spectrum analyzer, but they usually weigh a ton and are usually quite expensive.

About three years ago, I ran into quite a deal on a handheld spectrum analyzer that truly fit into my hand – unlike so-called “handhelds” that require both hands! Manufactured by Thurlby Thander Instruments, and virtually unknown here in the States, it’s distributed by well-known Newark Electronics, under the AIM-TTI brand (although, the actual unit is still branded TTi).

There are two models offered and I’ve had a chance to try both. The PSA1301T covers 100 kHz to 1.3 GHz ($1,500) and the PSA2701T covers 1 MHz to 2.7 GHz ($1,950). The leather case, which I recommend, runs $137.

In this review, I’ll cover the PSA2701T, which I have used for a couple years now. The PSA1301T is similar in specs; mainly the frequency range is different.

This is one clever little design. If examined closely, you’ll discover the entire user interface – controls and screen – is actually an embedded Palm TX PDA! By opening a couple side latches and lifting off the top cover, the PDA simply unplugs from the base unit. The PDA includes all the usual Palm applications, including Wi-Fi, so once you’re done measuring EMC, you can use the unit to check email and browse the Web! The product even comes with the original packaging and accessories for the Palm.

The spectrum analyzer circuitry resides in a fully shielded base section with an SMA connector for the RF input.The Palm uses custom software to turn the unit into a fully-featured spectrum analyzer. The unit even includes AM/FM demodulator circuitry and an earphone jack at the top for evaluating potential commercial ambient signals.There are a few key hardware controls, but most are touch-sensitive soft-keys.

Here are the key specifications:

★ Frequency range: 1 MHz to 2.7 GHz (100 kHz to 1.3 GHz for the PSA1301T)
★ Resolution bandwidths of 15 kHz, 280 kHz or 1 MHz (PSA1301T lacks 1 MHz)
★ Can read out in dBm or dBuV
★ Can enter frequency limits of “center-span” or “start-stop” ranges in 1 kHz steps
★ -96 dBm typical noise floor at -20 dBm reference level
★ Sweep modes of normal, single, peak hold and average
★ Zero span mode with AM and FM demodulation (1/8” earphone jack)
★ Two variable markers that read out either absolute or differential values
★ Marker “peak search” and peak tracking
★ Reference waveform display in a contrasting color
★ Programmable limit lines with limit line editor and store/recall
★ Unlimited storage of waveforms, setups and screens (can store to SD card)
★ 3.5-inch TFT touchscreen (64,000 colors)
★ Display resolution of 320 x 480 pixels (graticule area is 320 x 300 pixels)
★ Data transfer to a PC for analysis, documentation or printing (via SD memory card)
★ Battery operation of about 4 hours (includes AC power adapter/charger)

Troubleshooting with the PSA2701T and attached probe is fast and easy. No heavy instruments to lug out to the measurement chamber or open site and no line cords to plug in. Just turn it on and go! I found I can quickly zero in on an emissions issue, even during characterization or pre-qualification testing.

Emissions can be recorded via screen shots (bmp format) or tables of comma-delimited (or separated) variables (csv), which may be saved and imported into your favorite spreadsheet.What I especially like is the unlimited number of instrument setups I can save. Favorites of include 1 to 30 MHz for conducted emissions, 30 to 200 for low-frequency emissions, 100 to 500 MHz for a lot of my typical troubleshooting and 2.4 to 2.7 GHz for Wi-Fi and Bluetooth sniffing.

Screen capture of a Wi-Fi (violet) and Bluetooth (green) signal. Peak hold was used to allow the spread-spectrum signals to “fill in” the signal envelope. The Bluetooth signal was saved as a reference waveform.

The unit is sensitive enough with the larger 2-turn Beehive loop probe I recommend, that a preamplifier is usually unnecessary. By attaching a probe directly to the analyzer, you have the perfect handheld emissions detector! However, for some signals, such as some current probes or smaller loop probes, additional amplification may be required. I use the low-noise Mini-Circuits ZX60-3018G-S+ amplifier module as reviewed in a separate document, “Low-Cost Wide-Band Preamplifier” on my Web site (under Technical Papers). This amplifier module covers 20 to 3000 MHz with a gain of 18 to 23 dB and noise figure of 2.7 dB. Beehive Electronics also has a low-cost model 150A that covers 100 kHz to 6 GHz at 30 dB gain. I’ll be posting some info on this in the near future.

While the unit lacks the standard EMI bandwidths (for example, 200 Hz, 9 kHz and 120 kHz) or quasi-peak detection, I don’t find this to be a limitation during the troubleshooting process. What you’re typically looking for is “how much leakage is there now, and how much is there once I apply this fix?” Once the fixes are implemented, that’s when it’s time to measure your product in a semi-anechoic chamber with the proper measurement equipment as specified in the appropriate standards.

Closeup showing the Beehive probe and analyzer showing emissions leakage from a seam.

The analyzer includes a number of very handy features for general EMC troubleshooting. Features like markers, peak search, averaging, peak hold, waveform memory, amplitude scale in dBm or dBuV, screen capture and instrument setup memory – not to mention extreme portability – are found only in the higher-priced units. This is a very powerful tools for the EMC engineer. Using this low-cost instrument to perform the initial troubleshooting prior to moving the product out to a compliance test facility will save both money and time. This truly handheld spectrum analyzer may be purchased for about the monthly cost of renting a bench-top analyzer. Highly recommended.

Web site: http://www.tti-test.com/products-tti/rf/spectrum-analyzer.htm

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Spring 2012 EMC Seminar Schedule

Posted on February 22, 2012 | Leave a comment

Public Seminar Schedule
We’re continuing to work on our 2012 public seminar schedule. Of course, I’m available to book your in-house seminars as you require!

EMC Design and Troubleshooting Workshop (2 days) – Longmont, CO, March 20-21, 2012
Hosted by EMC Integrity Inc., 1736 Vista Dr., Longmont, CO
Details: $1,295. Includes seminar notes, lunch and snacks.

Download brochure

EMC Design and Troubleshooting Workshop (2 days) – Gaithersburg, MD, May 15-16, 2012 
Hosted by Washington Labs, 7560 Lindbergh Dr., Gaithersburg, MD 20879
Details: $TBD. Includes seminar notes, lunch and snacks.

Download brochure – will be available soon…

Seminar Content:

Day 1: EMC Theory (Part 1) – Attendees will be introduced to units of measurement, time and frequency domain, differential and common mode currents, radiated emissions, ESD, PC board layout and shielding/bonding. Day 1 is crucial to understanding the Day 2 presentation, as it will also include demonstrations of many of the basic principles.

Day 2: EMC Theory (Part 2), plus Bench Top Measurements and Troubleshooting – We’ll continue with ESD and system design, followed up with practical tools and techniques that can be used for pre-compliance measurements as well as troubleshooting EMC problems in a more formal setting. We will include the popular “Top Ten EMC Issues” and will demonstrate several probing and analysis techniques that will identify EMC issues quickly. Several case studies will also be described.

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