Shielded Bulkhead Connectors Mitigate Ambient Emissions for Military & Avionic EMC Testing

Most Military/Avionic EMC test programs include radiated emissions testing.  This is the test method where the unintentional emissions which 

Figure 1 – Shielded bulkhead connectors

 

radiate from a piece of electronic equipment under test are measured with an antenna and spectrum analyzer and plotted against the required limit.  One issue which must always be dealt with during radiated emissions and other EMC test methods is control of the ambient emissions.  Ambient emissions such as television, radio, and wireless signals are attenuated by the shielded EMC test chamber; however, they can travel into the chamber on the cables which connect to support equipment outside of the chamber. 

If ambient emissions are not mitigated, they can appear as an EUT failure during testing.  One way to deal with this issue proactively is to use shielded cables with shielded bulkhead connectors or feed-through capacitors (Figure 1) to decouple the ambient signals to ground.  

In Figure 2, note how the ambient emissions exceed the Army ground limit at 28MHz when the power and signal 

Figure 2 – Ambient emissions using an oversize hole

 

cables pass directly through an oversize chamber hole.  In Figure 3, see how the ambient emissions have been attenuated below the noise floor of the measurement system, due to power cables connecting through 1uF feed-through capacitors and the shielded serial cable connects through a wall-embedded bulkhead connector. 

Utilizing our machine shop, steel panels, and a variety of connectors, MET Labs provides custom bulkhead panels for all military and avionics test programs, at no additional cost to the customer.  

Similarly, some electronic equipment requires water as part of its normal mode of operation, such as aircraft 

Figure 3 – Ambient emissions using shielded bulkhead connectors

 

galley equipment or water-cooled high power equipment.  MET has installed regulated water at its Military/Avionic EMC test chambers using bulkhead fittings that maintain the shielding effectiveness of the chamber.  MET has a variety of hoses and fitting available and trained staff to provide professional hookup to the equipment under test.  This too is a free service provided to all MET customers. 

Visit our quote center for an immediate EMC testing need, or Ask Pat, our resident electrical product compliance expert, for questions relating to chamber pass-throughs for water pipes and power & signal cables.

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