The basic principles
Fluctuating voltages in a conductor of any type create radiated electrical waves while fluctuating currents create magnetic waves combining into electromagnetic (EM) waves. Every type of electronic equipment “leaks” some EM fields to some degree.
The electronic activity at radio frequencies in equipment causes radiated EM fields to be created at those frequencies which we call RF fields.
Radio, TV and Radar transmitters can emit very powerful fields. Arcs and sparks at electromechanical contacts, electrostatic discharges and lightning will also cause transient RF fields.
When conductors such as cables, connectors and PCB tracks are exposed to RF fields, current and voltage “noises” are coupled into them. Shielded cables and their connectors will help but there is always some coupled noise. Sufficient levels of coupled RF noise can cause errors or malfunctions in an analogue or digital circuit or switch-mode power converter.
Electrical and electronic equipment can also be particularly vulnerable to electro-static discharge (ESD) from people’s fingers or metal objects.
When to test for EMC compliance
RF emissions are increasing and RF susceptibility is worsening. Higher clock and power switching frequencies increase emissions but at the same time the trend to operating chips on lower DC voltages worsens immunity.
Identifying the potential for malfunction with rectification at the design stage will significantly reduce the cost of modification, and the resulting delays, at the post-testing or pre-production stage.
Close field probe testing
Failure to use good EMC engineering techniques in PCB design and layout invariably increases the manufacturing costs. Close-field probing is a low-cost EMC testing technique that will allow the design engineer to identify the potential for malfunction due to RF noise and ESD at an early stage.
Designers can use close-field probing to test for RF immunity as soon as they have “bread-boarded” a design. EMC performance can be improved by modifying the circuit board, moving components, re-routing PCB tracks and experimenting with board-level filtering
Instruments to probe
Engineers can become quite adept using close field probes if they use them frequently. However, each design of probe behaves differently so expertise with one probe may not translate quickly into expertise with another. There are safe and sure ways to locate the cause of an RF or ESD immunity test failure.
A variety of probes and instruments are available which can speed up the fault location process with varying success.
It is possible to construct a wide range of close-field probes. However, it is difficult to make comparisons between the different results of differing designs. The engineer will need to explore the advantages and disadvantages of the various options.
• Close-field probes can be made small to provide greater accuracy in pinpointing the cause of the problem.
• A magnetic field probe – a “shorted turn” shielded from electric fields, sometimes called a “loop” probe. This probe would be improved with a CM choke or ferrite toroid.
• An electric field probe – a very short “whip” antenna. This probe would also benefit from a CM choke or ferrite toroid
• A simple DIY unshielded magnetic loop probe. A rectangular probe can be more convenient for testing flat items.
David Goodway works for dB Tech