Filter connectors are often a misunderstood product for many components engineers. The lack of understanding is often a result of the fact that filter connectors seldom make the “What’s New” headlines and because many engineers typically don’t have a need for them. But those who do are certainly glad they are available!
Filter connectors are an invaluable tool in the world of electromagnetic compatibility (EMC). As such, designers and engineers primarily responsible for EMC will most often encounter the need for this possible solution to their problems.
Back in 1983, when the FCC Part 15 rules came out, and many electronics manufacturers (especially the computer folks) scrambled to figure out how to make their products compliant or risk it not going to market, the problem was most often referred to as EMI because interference was the big issue. Now the broader term EMC is used and applies to nearly every product in nearly every industry. So depending on who you work for, in addition to FCC Part 15 you will encounter EMC requirements based on MIL-STD-461, CISPR, FAA (AC20-136), and others from NASA, Boeing, Airbus, medical device or automobile manufacturers – and that’s just the North American standards. Seek out the European equivalents and you will have a pile of CE directives hidden in all those IEC specifications.
Designers must ensure that their product does not interfere with other electronics while also tolerating some specified level of exterior RF (thus the concepts of interference and susceptibility were rolled into one, and called compatibility). Controlling EMC boils down to three methods: shielding, filtering, and most important…. minimizing the energy and frequency spectrum that needs to be contained.
The proliferation of personal electronic devices (tablets, smart phones,
e-readers, GPS, etc), has been a two sided coin. These compact handheld devices are seldom connected to another piece of equipment due to their wireless capability – reducing input/output connectors and resulting in a multitude of EMC challenges. But the abundance of such RF emitting devices around equipment that is interconnected necessitates rigorous attention to EMC. You can quickly get an appreciation for this, when you consider medical electronics in hospitals, test or process control instruments in a factory, and the ever changing rules aboard aircraft. Then there’s all those new electronic devices carried or even worn by military personnel or emergency response workers.
Over the years an extensive science has evolved helping circuit designers minimize the need for filtering and shielding through careful design and layout of components in their circuits. But ultimately, all final designs will utilize all three methods.
This article focuses on the filtering method. Filtering simply means the control of the frequencies, the energy that they contain, and limiting where they go. Components that are frequency sensitive are capacitors and inductors. Filter connectors integrate capacitors, inductors, or both, inside a connector. Most often the connectors of interest are the input/output connectors, since those are not only the transition from the device to the outside world, but they are also the ‘antennae’ for concerning frequencies both coming and going.
As a result, the most popular and most filtered types of connectors are D-subminiature and their related families (which actually originated in MIL-C-24308), several circular types like MIL-DTL-38999, MIL-DTL-83723, and aviation industry specific ones like the ARINC 404 and 600 connectors. Typically, such use is for professional equipment, not consumer products.
The capacitor is the primary filter component, typically of a special construction that lends itself to fitting into the tight confines of a multi-pin connector. One lead is connected to
a contact the other to ground. The ground ultimately is connected to the shell and the device enclosure. While some filter connectors use simple ‘chip caps’ mounted on a substrate configured for the contact layout, the higher performance filter connectors use either tubular ceramic capacitors, or ceramic planar arrays. The very high end filter connectors contain multiple capacitors and ferrites to create a Pi type of filter.
Filter performance can range from a few dB (ferrites, chip caps) of attenuation, to 80 or more dB when used in Pi or higher order filtering, in a properly shielded and grounded connector design.
The key to a successful design is to only pay for what you need. Understanding your needs, the methods available to meet the specification and effective implementation will provide a marketable product.
So what makes the use of filter connectors desirable? If you have to use just one word – performance – they work. Key reasons why they offer advantages:
Filter performance far exceeds comparable filters placed elsewhere in the circuit. There’s a lengthy engineering explanation of why, but a 60 dB filter inside a connector, might only provide 15 to 30 dB if placed somewhere on the PCB behind the connector. While not all designs need 60 dB, some modern electronics do, leaving a filtered connector as the only EMC choice.
Higher frequency effectiveness, while related to #1, occurs due to more compact packaging, better shielding around the filters, and especially because of lower impedance ground connections. The rapid expansion of ‘wireless’ devices has made the need for high frequency control much more important than it was even 10 years ago. Useful filtering above 1GHz. is common, and necessary.
Space. The drive for compact designs might welcome component removal from the PCB and hide them inside the connector.
Component reduction. A 25 position D-sub, when filtered might remove 25 or more components from the PCB, when replacing an ‘on-board’ filter design.
Flexibility of EMC design and performance. A filter connector can easily be changed for one of different filter properties. Some filter connectors are ‘add-on’ adapters, which can be used on location specific installations.
It has been said that the use of filter connectors indicates that other aspects of the EMC design process have failed. Theoretically that might be true, however not all designs and not all markets have the ability to fully utilize only the alternatives. There are many high-performance, highly critical applications that do not have the development time and cannot risk avoiding the cost of a filter connector. Price has typically been a perceived downfall of filter connectors.
Filter connectors have a broad range of performance and prices. Higher volume, commercial filtered D-subs, start in the $3.50 and up range. Deliveries can range from stock, to 6 weeks. At the other end of the spectrum, aviation types, like ARINC 600 can be $1000 or more and take 18 weeks. The shock factor of such price and delivery will depend on what industry you work in, and your cost alternatives to meet EMC.
There will always be a market for the effective use of filter connectors as designers match their needs and apply cost effective solutions that work. Many $1,000 electronic boxes and $1,000,000 electronic systems would not meet EMC needs without the use of filter connectors.
Filter connectors continue to meet a specialized need in electronics design. Because each EMC situation is unique, it is important that designers work with filter connector suppliers to optimize their selection. Suppliers who make filter connectors a focus product will generally be most capable of providing superior technical support in the early consultation stage and will work to provide the designer the right product, at the right price, to meet his need.
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Fred Kozlof is currently with CONEC. Fred was involved with commercial filter connector design back when the FCC regulations were implemented! |
