EMC
Lead times of the various components used to filter out unwanted electromagnetic noises range tremendously
most common sources of audio frequency noise are: unwanted hum from power supplies, audio processing equipment (power amplifiers) and FMradio transmission. Radio frequency interference sources are: high frequency circuit signals (microcontrollers), wireless communication, and industrial and medical equipment.
Increased need The need for EMC has grown exponentially in recent history due to the proliferation of wireless devices, increasedmicrocontroller based products, and lower operating voltagesmaking the equipmentmore susceptible to ESD events and various other transients. Several national and international agencies have written EMC directives to promote compliance, performance and standardization. Various agencies have written guidelines to help the design engineering community develop compliant product particularly in highly regulated industries like;military, transportation,medical, and communications. The agencies range fromthe International Electrotechnical Commission (IEC 61000), FCC part 15, GR 1089, European EN 50 & EN 55 along with German directives VDE 0871 & 0875 as well as others and are solely in place to standardize the proper amount of emissions and test and measurement for products that will be sold into their jurisdictions.
As there are varying levels of emissions, there are also varying
levels of solutions that engineers apply based upon the regulatory guidelines for their end products and targeted verticalmarkets. The lowest level electromagnetic (EMI) filtering is accomplished with ferrite beads. Ferrites are an optimal quick fix as they dissipate the unwanted EMI noise through heat; therefore no access to ground is necessary. Ferrite beads are a great choice for filtering for applications that are under 5db of noise.
The next level For higher speed applications ormore than 5 db of rejection, the next level of electromagnetic (EMI) filtering involves surfacemount EMI filters. These can be referred to as a feed through filter as they were originally developed froma single feed through capacitor. When a filter is developed based on a capacitor only, it is referred to as a C type filter. Whenmore noise suppression is required a very basic combination of Resistors (R) and capacitors (C) or a RC circuit is the next step.With these basic components an engineer can create the twomost basic types of filters: low-pass filter and high-pass filter. Fromthere,more advanced levels of filtering can be accomplished by introducing an inductor (L) to create a LCR circuit. Different configurations of inductors (L), capacitors (C), and resistors (R) are housed inside ametal casing that provides shielding in noisy electronic environments. These are known as feed through or bulkhead filters and depending on the
26 | September 2010
component arrangement, can create a Pi, LC or T circuit / filter. These are among the highest level solutions for EMI problems. Most emission applications are compliant using these basic components in a circuit to filter out unwanted noise.
TTI’s Jeffrey Ray
RF filtering Radio frequency (RF) filtering is equally important and has a similar set of components to ensure compliancy to the appropriate regulatory agencies. There are three basic technologies of RF filters: multilayer filter, dielectric filter and SAWfilters – many of these filters operate on ceramic and piezoelectric technology. Additionally there are two primary kinds of RF filters: diplex filters (send or receive, but not both) and duplex filters (send and receive simultaneously). These RF filters can be designed to either allow a specific radio frequency band to pass into a device (bandpass) or keep unwanted ranges of RF signal out of a device (band elimination).Multilayer, dielectric and SAWfilters can be either diplex filters or duplex filters. Multilayer filters are the most cost effective and provide average levels of RF rejection. Dielectric filters provide good rejection of unwanted RF signals and have the highest power handling capability while SAWfilters provide superior levels of RF rejection and are significantly smaller and offer a very low profile for low board height requirements.
For the myriad of issues above, it is clear to see how TTI can
bring value to our customers and suppliers on EMC. Lead times of the various components used to filter out unwanted electromagnetic noises range tremendously. However, TTI has the world largest available to sell inventory of resistors, capacitors, inductors and ferrites to build your own custom EMC design. If you need to purchase an off the shelf solution, TTI is also a leader in fully assembled EMI/RFI filters, SMT EMI filters, SAWfilters, multilayer filters and dielectric RF filters.
Jeffrey P. Ray is vice president, productmarketing, Honeywell, discretes, EMC & circuit protection for specialty distributor TTI Inc.
www.ttiinc.com www.electronics-sourcing.com
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