Jonathan Newell visits filtration specialist MPE to find out how military grade electrical filtration is becoming a commercial necessity

Suppressing the surge I

f you’ve ever owned a car built in the 1970s or earlier, you were probably only able to listen to the radio in dry weather or suffer interference from the windscreen wipers. Even today, your computer screen can become speckled when

the table lamp is switched on if there is poor cable filtration. In both of these examples, the interference is low

grade and very easily suppressed with a low pass filter such as a ferrite core. However, in more critical applications such interference can happen at higher currents and across a very wide range of frequencies, with severe consequences. In these cases, a lump of ferrite isn’t enough. To find out more about filtration on a grander

scale, I recently visited MPE at its factory in Liverpool and met the company’s director, Paul Currie.

RADIATED VS CONDUCTED EMISSIONS Careful design of electronic systems, the positioning of antennas, selection of the right component values, quality of components used and the use of shielding are all important factors in preventing radiated emissions and protecting against them. Conducted emissions happen when radiated

signals couple with the conductor and send unwanted interference down the line. An important difference between the two is that radiated emissions reduce in intensity over distance, whereas conducted emissions do not. Wherever a cable emerges from a shielded

environment, it is vulnerable. An electromagnetic pulse, such as that caused by lightning, can couple

22 /// Testing & Test Houses /// June 2019

level of 80db to 100db across the entire frequency range from 14kHz to 40GHz. This requires the filter to be designed differently,” he says. Design differentiation is what MPE is

particularly good at. As Currie explains, filters are all essentially boxes of “Ls and Cs” (Inductors and Capacitors) but the expertise lies in their values, circuit configuration, manufacturing technology and containment. “The box we put them is crucial. There can be no

leakage of RFI, it has to be a fully EMC sealed faraday cage. With the kind of frequencies we handle, even minute breaks in shielding continuity due to, for example, poor soldering, has an impact on quality,” explains Currie.

to the cable and through to the shielded environment. To stop this, filters are used to remove the interference from within the conductor.

A MILITARY HERITAGE With his military background, it was natural for director of MPE, Paul Currie, to target the defence market for his technology when he co-founded the company. This is a market with technology that is a highly vulnerable target requiring specialist filtration to protect critical systems over a wide frequency range. As Currie explains, off-the-shelf filtration

products can be obtained cheaply that can protect a range of frequencies from 100kHz to 100MHz with an attenuation range of 40db-60db. With these filters, the attenuation lessens as the frequency increases so they’re not consistent over the whole range. “Our customers demand a consistent attenuation

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