EMI can be any unwanted electrical or electromagnetic energy that causes undesirable responses, degraded performance or failure in electronic equipment. Radio Frequency Interference (RFI) is a more specific term often used interchangeably to describe the phenomenon. A filtered d-sub connector is widely recognised as a very effective way to reduce noise either entering or exiting an EMI enclosure because it can often be incorporated into a product late into the design process; even after emission problems have been detected. Noise induced on a signal line within the box can be removed as it passes through the filter D, hence preventing radiation to adjacent equipment. Filtered dsub connectors are also used to provide system immunity from external noise sources. Filters integrated into D-sub connectors to achieve a reduction in conducted EMI have evolved over the years using a variety of elements and materials to achieve varying degrees of noise suppression. These filters are broadly identified as low-pass filters. A low-pass filter allows lower frequencies to pass through the filter with minimal reduction in signal strength, while attenuating signals with higher frequency.
A filter element achieves this by essentially becoming a variable resistor connected between the signal lines and ground. The resistive value is inversely proportional to the frequency of the signal it is conducting. As the frequency increases, the resistance of the filter decreases, providing a lower impedance path to ground. The capacitors strip off the interfering noise from the signal as it passes through the filter device whilst diodes are used to clamp the voltage below a certain value, thereby protecting the electronic circuitry.
Filters can selectively attenuate specific frequency bands by introducing controlled impedance mismatches into the line. A low-pass filter effectively strips higher frequencies to ground while allowing desirable lower frequencies to pass through the filter. Filtered connectors are typically bi-directional, in that they can attenuate noise originating from either side of the filter, an important advantage when considering immunity.
Many different configurations of filters have been developed to address a variety of circuit requirements. The simplest design is the C or feed-through filter which consists of a single capacitor inserted between the signal line and ground. As the conducted signal frequency increases, the effective resistance of the capacitor decreases, and therefore shunting higher frequencies to ground. The result is a filter that passes lower frequencies while attenuating higher frequencies.
The so-called C-filter is arguably the common standard. In this single-stage low-pass filter, each contact of the connector is connected with a capacitor to the connector housing (ground). Other options that can be used depending on the problem to be solved are the two-stage LC and the three-stage PI filters (C-L-C), which are also available in various connector configurations in the standard product range.
With a large differential between operating and interference frequencies, a single C-filter is often a sufficient solution for the EMI problem. If the differential is small, multistage filter configurations can be used. A C-filter suppresses the interferences from the “3dB point” by approx. 20 dB, an LC filter with approx. 40 dB and a PI filter with approximately 60 dB per decade.