Electromagnetic interference (EMI) is a headache inducing but often unavoidable issue in many system designs. This problem is ubiquitous and extremely harmful, and its impact will become more severe as the working frequency increases. EMI can be radiated through air, conducted through signal and power lines, and injected into circuits, or used as antennas to emit it again.
If a product generates or radiates EMI (i.e. "interference source"), it may interfere with the normal operation of nearby systems, fail compliance testing, and be prohibited from being launched. On the contrary, if a product is in the opposite role and becomes an intentional or unintentional recipient of EMI (i.e. a "disturbed object"), it may experience unexplained intermittent faults, failures, and unstable performance.
The impact of these issues is wide-ranging, ranging from being a bit comical like my wireless bicycle speedometer, to potentially life-threatening situations such as airplanes or hospitals, and causing huge losses in production lines. My speedometer operates in the 432 MHz frequency band, and for some reason, on a 100 yard stretch of road between a remote house and nearby Amtrak tracks (with a 20 kV overhead line above the tracks), the reading consistently shows between 65 and 85 MPH.
How to minimize the impact of EMI
Reducing or eliminating EMI sources and their effects can be both simple and complex. The basic steps include sufficient grounding, comprehensive shielding, reasonable bypass, and of course, the use of filters. In addition to these steps, there is usually the "Pareto Principle": eliminating 80% of interference requires only 20% of effort, while eliminating the remaining 20% of interference may require 80% of effort.
Any gap on the casing, such as the gap required for connector plugs and sockets, is like a door that allows EMI energy to pass through in both directions. However, if EMI is only caused by radiated energy, shielded connectors can solve the problem.
Decades ago, people began to address this issue, initially using coaxial cables and classic SO-239 and PL-259 female and male connectors, as well as BNC series connectors. However, these fully shielded RF connectors can only support one signal each and are not suitable for use with DC power supplies and non RF signals.
A good alternative is to "go back to the future" by using a connector type that once dominated communication links and other interfaces: the D-type ultra small (D-Sub) connector produced by companies such as Molex (Figure 1). Before the emergence of USB and parallel ports, engineers and many consumers used this 9-pin version (known as DB-9) connector as an interconnect device for the RS-232 serial protocol.
Figure 1: The widely used and durable D-sub connector and adapter series, with multiple contact numbers, electrical ratings, EMI filtering bandwidth, and physical termination methods; Pi filters can solve the problem of conducted EMI. (Image source: Molex)
USB and Ethernet have largely replaced RS-232, so this protocol is now mainly present in legacy systems and is rarely used in new designs; However, the D-sub connector has survived. There are many reasons why this type of connector is durable:
The seamless metal to metal design provides 100% shielding for wires.
The mechanical structure is sturdy and durable, and can be reliably locked between paired connectors using pins and screws.
There are multiple versions available for selection, including 9-pin, 15 pin, 25 pin, 37 pin, and 50 pin.
Provide multiple termination methods, including solder cups and direct insertion or right angle printed circuit board (PCB) pins.
When blocking alone is not enough to solve the problem
The shielding of D-sub connectors solves the problem of radiated EMI energy, but cannot solve the problem of conducted EMI. Therefore, Molex's high-performance D-sub Pi adapter and connector series for EMI filtering (also see Figure 1) has become an attractive solution.
These connectors have integrated EMI filters into their contacts, so there is no need to occupy additional space or add components on the printed circuit board. The grounding wire and insulated wire are located in the same connector, further saving space. They offer a variety of mechanical structures and terminal types to meet design requirements.
The built-in filter can prevent conducted EMI from passing through the connector, thereby reducing EMI in critical scenarios such as aircraft engine control, onboard radio, imaging equipment, processing equipment, and many other application scenarios.
The main adapter and connector features include:
Structure: The integrated die-casting shell and fully welded internal structure enhance mechanical and electrical performance, preventing malfunctions in high vibration environments. These connectors comply with the M24308 (MIL-DTL-24308) standard. Its glass fiber filled polyester shell also meets the UL 94 V-0 flame retardant standard.
Electrical endurance: These connectors can withstand up to DO-160 Level IV lightning strikes and AC transient environmental conditions in the onboard hardware environment testing standard.
Electrical filtering: Using a three element Pi configuration (capacitor, inductor, and capacitor), the filter can absorb high-frequency noise from power and signal lines. Its steep attenuation slope helps to suppress broadband EMI.
Feed in capacitors: To prevent unnecessary signal transmission at the interconnection point, feed in capacitors provide a low impedance grounding path. These capacitors can effectively reduce conducted radiation, especially in shielded enclosures where traditional capacitors perform poorly.
Inductive components (ferrite, toroidal coils): These components absorb high-frequency energy and dissipate it in the form of heat, minimizing accidental coupling.
The cutoff frequency of EMI filtering can be selected by the user, as these connectors provide a wide range of capacitance values, corresponding to a wide range of cutoff frequencies and related insertion losses (Figure 2).