by Alan Lowne, President, Saelig Co. Inc.
It pays to do preliminary studies before sending products off to the test lab for electromagnetic compatibility problems.
An engineer in a relatively small company usually must rely on experience and tribal knowledge to design a product that is electromagnetically compatible (EMC) with other equipment. Many designers and manufacturers don’t have the luxury of their own in-house RF test lab with an EMI-proof chamber equipped with expensive RF test-gear. That is why it is estimated that more than 50% of products fail the first time through an approved EMC testing facility. And failing is expensive. Retest costs are high and a retest may push back project schedules and market introduction dates.
However, pre-compliance testing has now become much more affordable. Such “early sniffing” can provide a good idea of problems before the fix gets expensive. Standards vary by country, but common EMC regulations for the U.S. are described in FCC Part 15, with subsections depending on whether or not the product is a consumer item. For Europe’s CE mark, EN55011 is the common standard, while some products have even stricter requirements.
Do-it-yourself, homegrown bench testing is becoming more necessary to head off problems passing the required emissions standards. It’s expensive to book an approved, certified test facility to ensure your new product meets EMI emissions limits. But it’s even more costly if your product fails and must go back to the bench to fix radiation issues, then return for a retest.
Fortunately, it is now quite inexpensive to purchase an RF spectrum analyzer with associated near-field probes or antennas, so you can get a first look at basic EMC/EMI problems before they do too much damage. Spectrum analyzers are now affordable—prices of quite sophisticated bench-top units have dropped dramatically in the last few years. They are even available in the form of a USB thumb drive, which can be connected to a Windows PC or tablet.
A set of sniffer probes—which look a bit like bubble wands for kids—can quickly find both the sources of problem radiation and help gauge the success of proposed fixes. The probes can disturb the field being measured, bringing added capacitance in proximity to an unwanted oscillator. Experience will reveal how useful and valuable this EMI tool can be.
Other useful tools for in-house testing include TEM (Transverse Electro-Magnetic) cells for radiated emission and immunity testing, and line impedance stabilization networks for conducted-emission testing of dc-powered equipment. Economical wideband amplifiers can boost the sensitivity of commercial spectrum analyzers or digital oscilloscopes using the scope’s FFT spectrum analyzer setting—but oscilloscopes are often not sensitive enough to provide much useful information.
Shielded enclosures—either bench-top metal boxes or quick-erecting portable EMI tents—are useful for keeping out ambient radiation and are well within the economics of most companies. The enclosure construction employs multiple layers of conductive silver/copper/nickel RoHS-compliant materials. These portable EMI test enclosures provide high RF/EMI attenuation for a variety of EMI-quiet applications. Their average shielding effectiveness is up to -98.9 dB at commonly used cellular and Wi-Fi frequencies. They erect quickly and are collapsible. External aluminum tent supports can be either fixed-frame or E-Z Up assemblies, so they can disassemble quickly when not in use.
Often, an internal or external vestibule is included as part of the design so personnel can enter and exit during tests without compromising the results. Tent options include size and orientation, use of through-plate connectors with integrated high performance filters for ac power configurations up to 100 A, communication connectors that include GigE and USB 3.0, and so forth. Tents even have their own ventilation and air-conditioning, a white ESD lining, and hardened, motion-detecting LED lighting.
Whether used inside shielded tents or on a bench-top, small sniffer probes for near-field testing can help discern EMI problems. You can often quickly isolate the source of EMI emissions using hand-held H-field and E-field probes. Near-field magnetic (H-field) and electric (E-field) probes can help home in on radiation problems in circuit layouts, cables and shielding. H-field probes use a conductive loop to detect magnetic fields produced by circuit board signals or switching power supplies. The probes produce a voltage corresponding to the magnetic field detected in front of the loop.
To find emissions on individual pins or PCB traces, use E-field probes in direct contact with circuit traces. It may be useful to evaluate emissions with more than one size of H-field probe. Kits often include several probe sizes.
Openings in enclosures and shielding cans can let emissions escape and cause current flow in surrounding metal enclosures. EMI gaskets or robust soldering techniques may reduce the offending signals. You can use an H-field probe to compare fields “before and after” inside an enclosure, or to gauge the RF energy a cable picks up from the source because of poor connector shielding. A near-field probe will help identify a cable acting as a radiating antenna. Current probes positioned around the cable can measure common-mode current that can cause unwanted emissions. A dipole antenna will reveal far-field emissions and is useful for checking trial fixes.
It can be challenging to modify a layout or circuit to eliminate an unwanted radiator. Typical methods of improving EMC include changing the clock speed, reducing rise-times and adding capacitors or inductors for filtering. Judicious changes to PCB layouts can also help with EMC. Usual approaches include moving a trace to a sub layer instead of the top or bottom layer, beefing up the grounding scheme, adding a shielding can, or redesigning the PCB as a multilayer board with ground planes on the outermost layers.
Another useful technique employs a low-EMI clock oscillator. This is one that slowly moves its center frequency by about 1% or so. An example is the Euroquartz EQHM series of low-EMI oscillators. They reduce EMI radiation using spread spectrum technology to modulate the output frequency with a low frequency carrier, spreading the peak energy of the output frequency and its harmonics over a wider bandwidth. This can eliminate the need for many expensive EMI protection aids such as PCB ground-planes, metal shielding, ferrite beads, RF gaskets and EMI filters.
A gap in shielding cans will also cause unwanted emissions. Changing the shielding configuration or soldering points can reduce the radiation. Typical problems that might be found include bypass capacitors too far away from ICs, poor power and ground tracks that allow ringing, and gaps in shielding. When RF emission emanates from power lines, adding an inductor or ferrite bead in the power track may be all that’s needed. Clock lines are often another emission source at low frequency, so it’s best to avoid long PCB clock traces on an outside layer.
After you’ve done all your homework, keeping unwanted emissions small and within limits, the product can go to the RF test lab with a high probability of success and assurances you’ve probably saved your company a lot of money!
Saelig Co. Inc.