Broad-Band and Narrow-Band Measurements

EMC specifications consider two kinds of signals (broad band and narrow band). Special care is to be taken to determine the nature of the signal if meaningful measurements are desired. Tests specified in MIL-STD-462 and in other EMC specifications to decide if there are narrow-band or broad-band emissions are not conclusive, and many times it is difficult to make a decision about the nature of the signal. The decision is less difficult to make when bandwidth effects are understood. This paper tries to show the implications of performing EMC measurements using different receiver bandwidths. It appears that broad-band measurements are valid only in the special case in which the signal has a continuous flat frequency spectrum; in the other cases their meaning is rather questionable. In the conclusion some remarks are made in order to clarify this difficult situation.