NORTH AMERICAEMI Filters & Components

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Phone: (814) 474-1571Fax: (814) 474-3110

EUROPESpectrum Control GmbH

Hansastrasse 691126 Schwabach, Germany

Phone: (49)-9122-795-0Fax: (49)-9122-795-58

AHA 4/06 ©2006 Spectrum Control, Inc.

www.specemc.com

FILTEREDCONNECTORS FORMILITARY/AEROSPACEAPPLICATIONSDESIGN GUIDE

www.specemc.com

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Filtered Connectors forMilitary/AerospaceApplicationsElectromagnetic Interference (EMI) is an unwanted signal that is either radiated or conducted to susceptible equipment and interferences with the per-formance of that equipment. The effect of EMI canvary from static to a complete malfunction of an aircraft navigation system.

Radar signals sent out from airports and aircraft carriers are a major source of radiated EMI. These signals canplay havoc on aircraft computers and cause a variety of dangerous scenarios to occur. The power supply within an aircraft is a source of conducted EMI. Conducted EMI is much easier to filter out.

Filtering an I/O connector prevents both radiated and conductedEMI from entering a black box, thus protecting the entire systemfrom interference. Filtered connectors are available in the samemechanical configuration as unfiltered connectors and every connector series is also available filtered. However, MIL-C-38999connectors have become the connector of choice in nearly all military and aerospace systems.

Choosing the Right FilterA noise suppression filter has to match the impedance of the systemit is to be used in. Figure 1 illustrates how to choose the circuit typerequired to match your system and maximize filter performance.

Typically, one should start to filter the signal 1 decade higher thanthe operating frequency. For example, for a 5-MHz data line, the filter cutoff frequency should be approximately 50 MHz. This willattenuate the noise without affecting the signal.

Construction of a Planar CapacitorThe construction of a planar capacitor is very similar to a chipcapacitor. Multiple layers of ceramic tape are stacked as shown inFigure 2, and an electrode pattern is screened onto the tape at a designated location.

After stacking, the capacitor is cut to size and holes are drilled. Theplanar capacitor is then fired at approximately 1300°C. Once fired,the capacitor is "silvered" by applying a silver composition to eachhole and outer grounding band. It is then fired again to bake on the silver composition. The product of this process is an array ofindividual capacitors assembled into a single ceramic disc (i.e. a 128-line planar capacitor has 128 individual capacitors). The discsthemselves range in diameter from 1/4" to 1-1/4"and are approximately 1/16" thick. Thisapproach results in a very durable compactpackage.

Advantages of Filtered ConnectorsFiltered connectors have three mainadvantages over discrete filters – weight,labor and cost. Assume that all lines of a55-line system require filtering. Assume aMIL-C-38999 Series I input connector and thatsome space exists for filtering within the system.Also assume that the system has a stringent weightrestriction and you are very close to that weight limit prior to filtering.

Replacing an unfiltered connector with a filtered one will addapproximately 2 ozs. to the weight of the box. On the other hand,an array of discrete filters will add 55 components to the box at anaverage weight of 1 oz. each, plus the weight of the plate they aregrounded to (which would be approximately 5-6 oz). Thus, the dis-crete filters add 60 oz. (3-3/4 lbs.) to the system weight.

The impact of labor is also significant. A filtered connector requiresthe same amount of installation labor as an unfiltered connector. But a filter array will require 110 additional solder joints (one on the input and the output of each filter). This adds approximately$100 to the cost of the box.

Construction of a Filtered ConnectorAlthough filtered connectors are custom products with limitless variations in capacitance per line, their design and manufacturing isstandard and repeatable. Because ceramics are susceptible to thermalshocking, ceramic processes use careful handling and even heating to decrease the risk of planar capacitor cracking. High temperature solder joints allow the customer to solder to the contacts withoutaffecting internal joints.

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Externally, a filtered connector looks the same as an unfiltered MILqualified 38999 connector. And the same seals and hard inserts provide the same environmental protection.

Mechanical spring fingers or compressed conductive rubber can provide electrical contact with the capacitor. This reduces the number of heat cycles or thermal shock the capacitor is exposed to.But solder’s metal-to-metal bond reduces resistance between contact

points and results in greater EMIfiltering. Long term reliability of a connector is greatly increased by using solder as opposed to amechanical joint. A solder jointwill not vibrate loose as easily or as quickly. In addition,mechanical joints are subject tometal fatigue due to constantcompression.

Comparison of Planar and Tubular ConstructionBoth tubular and planar capacitors can be used as EMI filters. Connectors using tubularcapacitors offer a large number ofdifferent capacitance values within

the same connector and better line-to-line isolation. But using planar capacitors can eliminate the need for an extension onto theback of the connector. This space traditionally would be used forEMI shielding, and now can be used to combine both EMI andEMP or lightning strike protection. It can also be used to enhancethe circuit’s performance (i.e. increase ferrite size or change from a PI circuit to a double PI).

A planarcapacitorreduces labortime. Forexample, a tubulardesign wouldrequire the

installation of 128 individual capacitors for a connector having a 25-35 insert arrangement while a planar design would require only onecapacitor for a feed through or two capacitors for a PI. Since installation is simplified, the chance of an assembly error is also decreased.

Spectrum Control Filtered ConnectorsSpectrum provides filtered versions of MIL-C-38999, MIL-C-26482, and MIL-DTL-83513 connectors. Custom configurationsand selectively loaded options are available in Pi or C circuits up to250,000pf. Typical configurations have pin or socket contacts witheither solder cup or pc tails. Custom Cabling can be added to anyconnector.

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Figure 1. I/O impedance and selectioncriteria

Figure 2. Construction of a planar capacitor.

To learn more about design considerations for EMI filteredmilitary and high reliability connectors, call 888-267-1195.