Understanding impulse bandwidth specifications of EMI receivers

When measuring emissions with an EMI receiver or spectrum analyzer, different signal types may be encountered. Their classification as narrowband or broadband is dependent on the signal's spectral distribution relative to the receiver's resolution bandwidth used for the measurement. It is essential to know which type of signal is being measured to avoid erroneous test results and the wrong interpretation of data. In this context, the knowledge of the instrument's impulse bandwidth is necessary to correctly determine the absolute amplitude value of an emission. The determination of the receiver's impulse bandwidth can be done with a series of measurements, but not simply by multiplication of its 3 dB or 6 dB bandwidth with a constant factor. Only the accurate knowledge of this parameter allows a meaningful comparison of the measured emission levels to a limit line and verification of a product's compliance with an EMI standard.