To measure near-field tests, you will need a spectrum analyser and measurement probes. While there are expensive spectrum analyzers on the market, you can still use a pocket RF explorer or a USB spectrum analyzer dongle in certain cases. You can either buy one of the many available near-field probes or make your own. Below is an image of two electric field probes. One has a pointed tip and the other has a round tip.
The rounded tip measures the electric field in a circuit over a larger area. The small pointed tip probe can be used to locate the exact area where the electric fields are generated once the area has been determined. These probes do not require orientation to measure in emi testing lab any direction.
Magnetic field probes use a loop to measure magnetic field. The image below shows two probes: one has a small diameter and the other has an enormous ring. The larger diameter ring measures magnetic field strength in the larger area. The smaller probe can be used for pinpointing the area in which electromagnetic fields are generated. These probes are sensitive to orientation. The readings will be taken in the same direction, so the probe ring parallel with the circuit will yield a better result.
A DIY probe is possible at a much lower cost than buying one from the market. Follow the below diagram to make a semi-rigid wire cable. Make a loop from the wire, then attach an outer shield to its inner wire. Then solder it to form a loop, as shown in the diagram. Near-field measurements can only give an idea of the problem, but cannot measure the emission. EMI/EMC testing labs use far field radiated emission tests. This can be expensive and difficult to set up. It is difficult to set this up in DIY labs. However, some EMC labs offer this service. Two types of far-field tests can be done.
Far-field measurement is dominated by the antenna. An antenna that is smaller and more expensive will be measured at a lower frequency will have a higher cost and antenna size. To get accurate field strength measurements, the antenna must be calibrated. To receive the maximum signal at a given height, the antenna must be calibrated. An antenna must move between 4 and 6 meters in order to obtain the maximum EMI signal. Low frequency will not be measured by the cheapest antenna. Below is an image of a PCB-based antenna that measures frequency between 0.6GHz and 10 GHz.
EMI testing is required for most electronic designs. So even if you are new to EMI/EMC (electromagnetic interference/compatibility), you need to understand what is involved and how to best prepare for a trip to the EMI test lab.
Good preparation is key to any trip. We will be looking at three phases: pretest, test, and after. Avoid potential problems and be prepared for unexpected situations. Most EMI tests fail the first time. Engineers should always have “Plan B”, and possibly even “Plan C” in mind.
A little philosophy is necessary before we can begin. Designers often take EMI failures personally. You shouldn’t take EMI failures personally. Instead, think verification. It should be a positive experience. We learn and improve, even grumpy EMC consultants.