Classification

Radio frequency emissions have to be controlled in general by legislative means, since the victim of RF emissions is typically a third-party radio receiver, whose user has no direct control over the source of interference. In the early days of interference control this would have been a domestic broadcast receiver, but in modern times there are many more services that need protecting. This is the reason for commercial limits applied through EU directives or other national legislation; for specific applications, such as aerospace or military, where the threat is to other equipment on the same platform and procured by the same systems integrator, limits can be applied through the procurement contract. For emissions, the phenomena to be limited can be classified as:

radiated RF

conducted RF, supply port

conducted RF, other ports

clicks/transients (mostly from domestic appliances)

This list covers most high frequency emissions phenomena of importance. The detail of which phenomena are regulated depends on the type of product – that is, which sector it is intended for, e.g. commercial, industrial, military, medical , transport etc. – and the expected environment in which it will be used.

A good example is demonstrated in this video. An LED light, typically powered by a mains-powered switched-mode power supply, can severely disrupt a radio receiver. In this set-up, the LED light is placed about 1 meter away from the FM radio, which, at the time, was playing BBC Radio Classics. A small log-periodic antenna was also placed next to the radio, and the antenna was connected to a spectrum analyser, which captured the noise. Even without the LED being switched on, in standby mode, the antenna and the spectrum analyser had already detected noise. When the LED was switched on, meaning the switched-mode power supply was loaded, the noise increased. As a result, the music coming out of the radio receiver was severely distorted, and one could observe an increase in noise on the spectrum analyser.

Frequencies

Principal frequency ranges are related to the expected radio receiver victims that can be found in the near environment. Table 1 below lists the frequency ranges that are regulated in the commercial (CISPR) and military (DEF STAN and MIL STD) standard requirements.

Table 1  Frequency ranges for emissions tests

Ports

Part of the classification includes the "port" through which interference is passed. A port is defined as a "particular interface of the specified apparatus with the external electromagnetic environment", and is usually classified as the enclosure, the DC or AC mains supply, and signal or I/O interfaces.

Although RF signals are present in the environment or generated by the product at many frequencies, the coupling routes into or out of the equipment vary and therefore the phenomena are treated separately over different frequency bands for the purposes of testing and regulation. RF emissions are classified into conducted and radiated, and for ease of measurement and analysis, in the commercial standards radiated emissions from the equipment and its cables, considered as a whole, are assumed to predominate above 30MHz and conducted emissions along the cables are assumed to predominate below 30MHz. Military standards have overlapping frequency ranges for conducted and radiated, but separate their radiated measurements into magnetic and electric field. Field strength levels are generally limited to a few tens of microvolts per metre, this being the "compatibility level" for typical radio receivers.

All high frequency measurements are made with respect to a ground reference plane. Although such a plane is rarely explicitly present in real life, for a standardized, repeatable test it is essential.

Detector types and bandwidths

A further distinction, which makes it difficult to compare measurements between commercial and military standards, is the specification of detector and measurement bandwidth. Because interference may be continuous, modulated or pulsed, it is necessary to have a detector which will respond in a predictable way whatever its nature; and it is also a requirement of the commercial emissions measurements that modulated or pulsed interference is treated to an extent more leniently. For instance, the quasi-peak detector is a peak detector with weighted charge and discharge times which correct for the subjective human response to pulse type interference. Interference at low pulse repetition frequencies is said to be subjectively less annoying on radio reception than that at high pulse repetition frequencies. This is not the case with military measurements, which use a peak detector and a set of decade bandwidths. The CISPR measurements use the quasi-peak and average detectors, and other bandwidths. The relationship between the detector responses and the repetition rate of pulsed interference is shown in the graph, and the bandwidths are shown in table 2. The peak detector will always return the highest value, whatever the nature of the signal.

Table 2