These EMC products are divided between achievement and verification. Modern electronics, especially the power variety, often provide high efficiency by switching. Fast transitions between the fully-on and -off states result in lower switching losses. Fast switching also generates EMI, which must be eliminated at its source or contained. Absorber material, shielded honeycomb air filters, and EMI gaskets are appropriate for these purposes.
EMC verification involves all four combinations of emissions, immunity, radiated, and conducted. One of the immunity tests often applied subjects the EUT to a high-voltage electrostatic discharge from an ESD generator gun. In contrast, very well-controlled conducted RF susceptibility testing requires a coupling-decoupling network (CDN). A gun and a CDN are included here.
Shielded rooms and anechoic chambers also are commonly needed for EMC evaluation because they isolate the EUT’s own radiated emissions from those in its environment. A small, portable, RF-tight enclosure and a new type of anechoic chamber absorber are featured. Also included is a line impedance stabilization network (LISN) used to evaluate conducted emissions.
Anechoic chamber absorbers
If you already have an anechoic chamber but its original absorber performance has become a limitation, you may decide to retrofit with DuraSorb model DSH-600H absorbers for 3-meter and 5-meter chambers or model DSH-1250H for 10-meter chambers. These nonhygroscopic polystyrene products combine the advantages of the company’s FerroSorb performance and the physical properties of the PS series absorbers.
DuraSorb absorbers are constructed using substrates with rigid, closed-cell foam, which are loaded volumetrically and uniformly throughout. Testing has confirmed that the product provides a smooth transition from the impedance of free space to the lossy ferrite tile base allowing use from 30 MHz to at least 40 GHz. In addition, environmentally friendly fire retardants make the pyramidal absorbers extremely fire resistant. The material meets the requirements of EN 11925-2, UL 94-HBF, DIN 4102-B2, and EN 13501-1 Class E. ETS-Lindgren
The Model STE2950 is a top-loading, RF-tight enclosure measuring 11.7-in. H x 8.3-in. W x 18-in. D and weighing 18 lb. Designed for WLAN and MIMO testing, the RF absorber-lined box features 90-dB isolation at 1 GHz and 3 GHz, reducing to 80 dB at 6 GHz. Electrical connections are made through a universal I/O connector interface plate. Dual 60-mm ventilation vents are standard, or fan cooling can be added.
For rack-mounted test systems, the STE2952 provides dual STE2950s mounted on a custom 19-in. rack slide. The STE2952C also uses two STE2950s, but premounted and configured in a custom shock-mounted road case suitable for field service trips. Ramsey Electronics
High-amplitude pulse CDN
The HSS-2 CDN is used to couple electrical surges onto unshielded symmetrical high-speed telecommunications lines such as found in Ethernet networks. This CDN applies pulses up to the maximum 2-kV 1.2/50 µs or 50-A 8/20 µs specified by IEC/EN 61000-4 through a 40-Ω resistance in series with a capacitive coupling element. As many as eight wires can be accommodated, so the common-mode pulses can be coupled to one, two, or four twisted pairs of ISDN or 10/100BaseT or 1000BaseT Ethernet. Because of the good decoupling, even with a 2-kV surge test voltage, the maximum voltage at the auxiliary equipment port is only 65 V. This means that additional decoupling is not necessary to protect measurement instruments.
Measuring 200-mm W x 200-mm H x 470-mm D, the 15-kg CDN supports a 1-A maximum operating current, withstands 100 V between pairs, and has an insertion loss of approximately 9 dB at 100 MHz. Teseq, Ametek Compliance Test Solutions
Thermal and EMI absorber material
CoolZorb 400 is a hybrid absorber/thermal management material that is used for EMI mitigation. It is positioned like a traditional thermal interface material between the heat source, such as an IC, and the metal chassis or heatsink. Because of the silicone gel binder that is used, the material is tacky and compliant so additional adhesive isn’t required.
As a thermal interface, the 2.0-W/m-K thermal conductivity compares with 0.71 for mica and about 0.3 for cast silicone rubber. Volume resistivity is 5.9E10 Ω compared to 1E12 to 1E14 for mica and 1E14 or higher for cast silicone rubber. As an EMI absorber, attenuation is about 10 dB/cm beginning at 4 GHz and increases with frequency to 13 dB/cm at 10 GHz. For both thermal and EMI applications, the filler particle composition improves performance.
CoolZorb 400 comes in a range of thicknesses from 0.020 in. to 0.130 in., is suitable for use from -20°C to +100°C, and passes the UL 94 V-0 requirements. Laird Technologies
Tuned frequency absorber
Resonant frequency absorbers, such as the manufacturer’s type MR11-0031-20 with pressure-sensitive adhesive backing, are tuned to provide as much as 20 dB of attenuation over a narrow frequency range. The specific performance depends on the amount and nature of the magnetic particle loading applied to the silicone base material. For type MR11-0031-20, the attenuation peak is >30 dB at ±10% of the 20.6-GHz nominal frequency. The material datasheet states, “MAST engineers can tune an absorber to any frequency from 1 to 40 GHz by simply changing the formulation and thickness.”
Type MR11-031-20 is a 12-in. x 12-in. dark grey silicone sheet 0.043 in. thick with an additional 0.005 in. adhesive layer. It is suitable for applications that encounter temperatures from -60°F to +375°F and satisfies flammability rating UL 94 V-0. Larger 24-in. x 24-in. sheets also are available (MR11-0031-01) with adhesive and without (MR11-0031-00). MAST Technologies
Textile EMC gaskets typically are easier to use than stainless steel gaskets, which until recently have been the only practical solution for applications involving temperatures above 70°C. A new textile gasket with silicone internal materials and conductive adhesive backing now is available for use up to 85°C.
The new gasket joins the company’s existing TPE foam-based textile gasket suitable for temperatures less than 70°C and a stainless steel gasket for temperatures higher than 85°C. Not only are the new gaskets lower cost than the stainless steel alternative, but the new textile product may allow for shielding levels from 30 dB to 80 dB, 8 dB to 10 dB higher than the earlier version fabric gasket, according to a press release. All three types of gaskets are intended for use on the company’s cases, subracks, and systems. It is anticipated that the higher temperature gaskets will find use in rail, defense, and medical applications where temperatures can extend to 85°C. Schroff, a Pentair brand
Honeycomb air filters
Air filters based on conductive honeycomb material provide EMC protection by operating like a waveguide beyond its cutoff frequency. For a signal to pass through a waveguide, the signal’s wavelength must be similar to the waveguide’s internal dimension. These fan filters all use 1/8-in. cell honeycomb material in a range of thicknesses. Approximating the hexagonal honeycomb shape by applying the standard equation for either a rectangular waveguide or a circular one yields a result of about 50 GHz—lower frequency signals will not propagate through the filter.
How the honeycomb material is joined internally as well as to the filter mounting flange also affects shielding properties. The manufacturer has developed a “blending” process for aluminum honeycomb panels that ensures good contact along all honeycomb joints. The latest improvement adds an EMI gasket around the mounting flange perimeter and is applicable to both the company’s Econo-Cell and Spira-Cell families of air vent filters. Spira Manufacturing
Quick-draw ESD testing
The Model 1000 ESD generator gun is intended for use in precompliance testing during product development as well as in quality assurance testing or manufacturing troubleshooting. Although not suitable for compliance testing to IEC/EN 61000-4-2, this product nevertheless has many advantages compared to much more expensive and cumbersome generators.
Specifications include both positive and negative 3-kV to 15-kV air discharge through 300 Ω. Pulse risetime is shorter than 20 ns, and because the generator uses a piezoelectric element, neither external power nor internal batteries are required. Life expectancy is about 45,000 cycles. The gun measures approximately 6.7 in. x 3.9 in. x 0.94-in. and weighs 7 oz. Bartek Technologies
Two-line LISN models 3810/2 and 3816/2 cover the 9-kHz to 30-MHz frequency range and facilitate use of a signal analyzer to make FCC 15, VDE 0871, VDE 0875, and EN 55022 conducted emissions conformance measurements. Both 10-A units include an artificial hand circuit and an earth line choke that isolates the EUT from RF ground while maintaining a safety ground connection. Only the Model 3816/2 has remotely switched test ports and a switchable high-pass filter that prevents overload. When the 3816/2 is operating in the remote switching mode, the selected line A or B is terminated in 50 Ω when power has not been applied, permitting noise floor measurement. ETS-Lindgren
