RF Solutions

EMC Introduction

Introduction

Electromagnetic interference is of major concern for OEM’s of electronic/electrical products particularly in communication, information technology, transportation, medical , military, and safety applications.

There are three forms of interference which are of concern:-

• EMI – Electromagnetic Interference
• RFI – Radio Frequency Interference
• ESD – Electrostatic Discharge

Due to the production of more sophisticated and complex electronic equipment, with electronic circuitry a device will produce a potential source of EMI/RFI or ESD therefore there are 3 factors to consider.

• The development of digital devices utilising small integrated circuits and microprocessors generating EMI/RFI and their susceptibility to the same interference generated from an outside source or device.
• The popular use of plastics has increased the problem, as plastic is transparent to frequencies and has no inherent shield.
• Government regulations require OEM’s (Original Equipment Manufactures) to limit the amount of EMI/RFI emitting from their device/products.

Explaining EMI & ESD

The two conditions know as RFI and EMI can be classified as EMR (Electromagnetic radiation). EMR is a phenomenon which occurs naturally as well as being man induced from radio waves, microwaves and electronic devices. As an everyday example of EMR the most visible is interference on a PC/TV Screen caused by an outside source and poor insulation/shielding. EMR has both electrical and magnetic properties referred to as; E fields=electrical, H fields=Magnetic.

The fields can induce current flow in a conductor and it is this ability to induce current flow which is of importance causing problems with devices/electronics. When EMR comes in contact with a device conductor, known in this situation as susceptors and includes power cables, PCB’s and various wire connectors of a device, it generates a current independent of the operating current by inducing flow in the susceptors and the PCB can receive and respond to these currents or signals just if it was receiving the intended operating current, whereby random RF’s would give electrical instructions.

ESD can be easily explained by the everyday experience of the shock you receive whilst getting out of a car, or from person to person after walking across a carpet, earthing out. This occurs due to a rapid flow of electrons moving from a charged object to another, whereby the potential difference between them is equalised. This natural occurrence can induce EMR into a conductor only for a split second, much in the same way as lightening effects radio and TV reception, but can have a devastating effect on the miniaturised circuitry which only requires a small amount of energy to be disrupted or destroyed as the ESD goes to ground.

EMR occurs naturally, produced by sun spots, radioactive decay and earth’s internal forces, but these are of no concern it is the man made EMR which is of concern, caused by devices that generate unwanted EMI. The frequency range that causes these problems, ranges upto Several Gigahertz.

The guide lines on these frequencies and standards used come from the following international bodies;

• FCC – Federal Communication Commission
• VDE – German Postal Services Testing Agency
• EEC – European Electromagnetic Compatibility

The basic EMC requirements for product compliance, are as follows and require to be fulfilled and monitored by enforcing legislation;

• Electronic apparatus is constructed so that any electromagnetic radiation generated does not exceed a level that impairs the performance of other devices.
• The apparatus itself must be sufficiently immune to the effects of electromagnetic radiation, to enable it to operate as intended.

The penalty for non compliance will result in the manufacture not being able to market their equipment in the European Community nor in the country of origin within the EC.

Aid to Product Compliance

Conductive coatings offer a cost effective method of ensuring product compliance within the EC, USA and FAR EAST, by reducing or preventing the EMI and can be used in conjunction with other products and shields protecting the susceptors, which cannot be entirely shielded using a conductive coating alone such as apertures, cables, ports, displays, keypads and so on. Coatings being used in the control of EMI can be defined in two ways;

• Setting the acceptable levels of EMR from the housing to the outside environment.
• Describing the immunity of equipment to EMR from outside environment.

The EC legislation on EMC controls the problem of interference at source. They give limits within, which the EMR from the device must be contained. These limits are described in terms of frequency range, field strength and measured distance.

This ability to be able to calculate and measure the limits with relative ease gives the device an advantage. Measuring EMR at source is more difficult but a properly shielded device will automatically be well protected from the outside EMR.

Controlling EMI Using Conductive Coatings

The following illustrations show the device assembly and how it is protected by a conductive coating from either causing interference from its electronic assembly or by susceptiblility to interference from an outside source.