A quick and easy EMI filter solution for meeting strict military and aerospace interference standards can save a lot of time and money, reports Sally Ward-Foxton.
In the final stages of testing for military and aerospace equipment, it is unfortunate but quite common to be faced with EMI (electromagnetic interference) and RFI (radio frequency interference) problems. Standards such as DO-160 and MIL-STD-461 are strict, and lab time for testing is extremely expensive, so getting through the testing as quickly as possible is critical.
If the design has failed to provide adequate EMI filtering on the board at this stage, a complex and lengthy redesign could be undertaken. Alternatively, a filtered connector could be used in place of a standard connector. However, filtered connectors are much more expensive and lead time on these parts can be very long (16-52 weeks is not unusual). The design would then have to be retested for compliance at that point, meaning more expensive lab time.
An American company has come up with a neat solution to this problem. Quell, based in Albuquerque, New Mexico, has been manufacturing its custom-made silicon rubber EMI filters, which fit inside military and aerospace connectors, since 1995. The company’s design places 0402 and 0603 components inside a rubber disc (see figure 1), with soldered metal wires connecting the parts together to make up the appropriate filter design. Holes in the disc fit neatly around the connector’s pins, with the wires protruding slightly into the holes to connect the filtering components to the appropriate pins.
“Inside the silicon are two wires soldered on each end of each component, and they protrude into the holes to make connection with the pin. Then we have two wires on the back that simulate a ground plane,” explained Scott Lindberg, Quell’s Director of Sales and Marketing. “So for a filter, we put a capacitor in parallel with the pin, ground it to the shell and it reduces the noise. We can take a standard connector and make it into a filtered connector in a matter of seconds.”
Capacitors, resistors, transient voltage suppression (TVS) diodes, metal oxide varistors (MOVs) can all be utilised, along with practically any other filtering component that’s small enough.
Connecting the filter components together is achieved using a pair of thin metal wires in parallel for redundancy purposes, and the wires are crimped – each is 30% longer than the distance it covers, so that if the filter is stretched or bent, the wires don’t break (see Figure 2). There is also an additional wire inside for strain relief. The filters can be stretched, compressed or bent without sustaining damage. Lindberg said they are typically at least as robust as the connectors they are installed in – there is no PCB or ceramic array to break. Because the components are embedded in protective silicon, they can’t dislodge or become damaged while in use. The silicon can withstand extreme temperatures and exposure to ozone, chemicals, ageing and UV.
To install, the rubber filter inserts are pressed gently into the connector by hand (see Figure 3). No modification to the connector is required and the connector and filter assembly is able to maintain the connector’s existing environmental seals, where present. The filter’s thickness is between 28-32 mils (0.7-0.8mm) when the connectors are mated - not enough to affect the mating of the connector.
“The filter is a little oversized, it stretches and compresses,” Lindberg added. “Just push it over the pins of the connector. And once the pins protrude through the holes, you mate it, and just like that, you can make a filtered connector. No need to wait 16 weeks [for delivery of filtered connector parts].”
Quell has recently added to its EE Seal range with a technology called EE Seal+, which uses a conductive silicon impregnated with silver to make the connections between the filter components.
“The conductive silicon gives us the advantage of lower contact resistance and lower inductance which improves the attenuation,” Lindberg said. “This material gives us better high frequency performance. We can also make an integral capacitor out of the conductive silicon which can be as small as 2 to 5pF to filter noise in the range of 20 to 40GHz. So really high speed, high frequency problems can be mitigated with this product.”
Peak attenuation for EE Seal+ is around 50dB and frequencies up to 40GHz can be mitigated. EE Seal+ also makes it easy to ground pins to the shell, such as coaxial shields in combo D-sub connectors and coaxial circular connectors.
Meeting customer needs
All Quell filters are custom-made to the customer’s specifications, to fit the individual connectors (see Figure 4). The customer passes information such as the connector’s part number, size, shape and pin configuration to Quell, along with details of the EMI problem, such as particular frequency bands, voltages (up to 2,000V), etc. Quell then design a silicon rubber insert with the appropriate filter elements, shorts or opens on each pin. Sample filters can be made fast enough to be shipped in 24-48 hours, well suited for designs that have failed EMI testing and are still in the lab, while production quantities take two to six weeks.
And while Quell’s modus operandi is being a last resort option to get designs through qualification, some customers design Quell filters into their standard connectors from the beginning, Lindberg added, as it typically works out 20-40% cheaper than filtered connectors. They can also be used as a fall back option which may or may not be needed. Designs can quickly be tested in the lab with and without the EE Seal filter, and if it’s needed, it’s already there.
“A lot of people design in filtered connectors from the start, because they don’t know if they are going to have a problem. But if you’ve got, say, a 26 week lead time, you better order it now!” he said. “The advantage we have is you can order standard connectors and if you have a problem, just call us. If it’s not needed, you’ve got away with it!”
The vast majority of Quell’s customers are in the military and aerospace sectors (they have received the Gold Composite Performance Excellence Award from Boeing and a 100% Quality Supplier Award from Lockheed Martin), but the company also works with customers in applications such as transportation and high end medical equipment.
“We’re a low to mid-volume supplier, our biggest customers order around 20,000 parts per year, but most are in the range 500 to 1,000 per year,” Lindberg added. “Another recent customer wanted eight pieces, and that’s fine too. Our minimum order is one.”
Lindberg makes a compelling case for Quell’s technology. Solving last minute design problems in 48 hours, versus waiting months for a redesign or filtered connector, is very attractive. Additionally, if you consider that adding a separate Quell filter is significantly cheaper and quicker than specifying filtered connectors in the first place, this solution becomes second only to on-board filtering. It’s no wonder these parts are widely used in commercial aircraft, heavy duty transport vehicles and military anti-jamming equipment.