Best Practice


A checklist

What are the tests?

Check out the tests required

EMC Basics.

These are best practice and summary documents prepared by dB Technology covering EMC basics and including a decision tree to decide whether or not testing is a requirement for your product. (PDF Document right click and save if required).

The RA webpages are a good general purpose resouce that reasonably covers most EMC Issues

Best Practice

In order to achieve compliance the following rules generally apply to products containing high frequency circuitry where noise is typically harmonics of clock frequencies:

All high frequency (e.g. microprocessor) circuitry should be contained within a shielded enclosure. This can be local at the source of high frequency noise and may include a local plane on the pcb as part of the enclosure, or a general enclosure that surrounds everything except cable exits and connectors. Or both!

Any gaps between panel fixings should not exceed 10cm (4 inches), or possibly less if the hf sources are over 50 MHz .

Any non-metalic contact (e.g. touching of painted, oxidised or passivated panels ) counts as an aperture. Positive means of making direct metal to metal contact should be provided at appropriate intervals. Earthing wires are not a substitute for direct metal to metal connection - they are too inductive.

All cables entering or leaving said enclosure should be either screened or filtered (including mains cables).

Cable screens must be bonded to the enclosure at the point of cable penetration or connector. Tail wires should NOT be used for connecting cable screens. Braid screen cables are significantly better than foil, and ensure the screen is well bonded using appropriate connectors. Ethernet screens and similar interfaces with isolation requirements should be capacitively connected to the panel using surface mount capacitors with as near to zero track length to the panel as possible. Discrete capacitors with 4mm leads are considered too inductive and the cable may as well be considered to be not screened.

When filtering, shunt elements should be taken direct to the enclosure at the point of penetration. Note that the general rule applies: that with radio frequency noise the lengths of component leads are significant - the lengths of the leads of noise shunting capacitors should be limited to 1mm or 2 mm if they are to work effectively at frequencies up to several hundred MHz. Usually ceramic capacitors are needed for shunting noise frequencies above about 10MHz. RF noise is typically common mode so treat 0V lines the same as signal lines.

NB. Although the above referred to high frequency circuitry and emissions the same comments apply to sensitive ANALOGUE circuitry and immunity.