Narrowband and broadband discrimination with a spectrum analyzer or EMI receiver

Spectrum analyzers and scanning receivers are widely used in EMI laboratories today. Their use for measuring both narrowband and broadband signals requires specific understanding of certain instrument and signal characteristics in order to correctly interpret the displayed results. This paper explains methods for the discrimination between narrowband and broadband signals and provides guidance for the proper operation of test instrumentation. In the field of EMC, the two main categories of signals encountered are of particular importance: narrowband signals and broadband signals. The International Electrotechnical Vocabulary (IEV) defines a narrowband disturbance as "an electromagnetic disturbance, or component thereof, which has a bandwidth less than or equal to that of a particular measuring apparatus, receiver or susceptible device". Consequently, a broadband disturbance is defined as "an electromagnetic disturbance which has a bandwidth greater than that of a particular measuring apparatus, receiver or susceptible device". This means that the classification of a signal as narrowband or broadband is determined by the occupied frequency spectrum of the signal under investigation, relative to the resolution bandwidth (RBW) of the instrument used for measurement. If the signal spectrum is completely contained in the passband of the IF filter, it is defined as a narrowband signal. The general definition of a narrowband and broadband signal is depicted in Figure 1. It is important to note that continuous wave (CW) signals are a specific case of narrowband signals, since they consist of only one spectral line which is within the passband of the intermediate frequency (IF) filter. This case is depicted in Figure 2 (right). If the occupied signal spectrum exceeds the bandwidth of the filter, the signal is considered to be broadband. This is the case for the spectra of pulses (which are coherent signals) and noise (non-coherent signals). This scenario is shown in Figure 1 (left). This paper presents various methods that are suggested for the determination of signal characteristics in EMC standards and literature. It also discusses their advantages and disadvantages. The presented material, builds on previous papers that addressed the measurement of impulsive signals and discussed test equipment parameters such as the definition of impulse bandwidth and the purpose of preselection. Therefore, this paper will defer to previous publications for details, as necessary. Narrowband and broadband signals can be generated by a variety of sources and usually represent different interference potentials for radio services. Very often an interference spectrum from equipment under test (EUT) contains both signal types. Since both signal categories require a different interpretation of the result measured with a spectrum analyzer or EMI receiver, it is essential to know the characteristics of a signal in order to correctly determine its' frequency and amplitude. In some cases, the characteristics must be known, in order to select the correct limit for the determination of EUT compliance. The measurement results displayed on these instruments are also dependent on some control settings, such as the sweep time and resolution bandwidth. Their impact on the measurement of signal parameters, like frequency and pulse width, must be understood to avoid erroneous interpretations of measurement results.