Radio frequency emissions or noise are among the most difficult problems to handle when designing with fast digital circuits. Problems do not occur only as noise radiated to the outside world. Handling noise issues internally in the system is equally important.
The tests are split into two different types; radiated emission and conducted emission. This split is mainly done to make the tests practical to implement and because conducted emission dominates in the low-frequency range, while radiated emission dominates in the high-frequency range.
Radiated emission is radiated directly from the system and its signal/power cables. This is high-frequency radiation, as a normal PCB is too small to be a good antenna for low frequencies. The EMC directive requires measurements in the range 30MHz to 1GHz. American FCC rules require measurements at higher frequencies for certain applications. Lower frequencies are measured directly on the cables.
The high frequencies will typically be generated by harmonics of digital oscillators and I/O pins. Note that the upper frequency generated by a digital circuit is not limited by the clock frequency of the device, but by the rise time of the signals. Lowering the clock speed of the system will therefore not lower the bandwidth of the noise, but will lower the power radiated at high frequencies. (Reducing the number of noisy transitions will reduce the total power of the noise.)
Conducted emission is measured on cables. The EMC directive requires measurements in the range 150kHz to 30MHz. Some test standards require measurements down to 9kHz. Noise in this frequency range is typically from switch mode power supplies and from the base frequencies of digital oscillators and I/O pins.
Long cables will, of course, also act as antennas for both low frequency and high frequency signals. But if the LF signals are damped sufficiently to be below the limits of the conducted emission test, the radiation from the cable will be negligible. It is therefore not necessary to measure radiated emission in the range below 30MHz.
Similarly, conducted HF noise on the cables will show in the radiated emission test. If the noise is sufficiently damped to be below the limit for radiated emission, the conducted noise on the cable will be negligible.
Test setups and limits for different applications are given in various standards issued by the International Special Committee on Radio Interference (CISPR). CISPR 22, for instance, covers information technology equipment.