The existence of monochromatic structures in ULF or VLF magnetospaheric emissions has often been questioned, an in some cases it has been demonstrated that the apparent frequency structure of these emissions was due to the method of analysis. We present here a theoretical study of the apparent frequency-time structures which should result from different frequency analysis techniques applied to two kinds of signals. Signals of type I consist of two or more gliding tones (whose frequency varies linearly with time) closely spaced in time. Type II signals consist of two or more quasi-monochromatic emissions of finite duration, slightly displaced in frequency and time. We show that when very narrow or very wide filter bandwidths are used, similar frequency-time patterns may be obtained with both signals. These patterns are described, as a function of the characteristics of both the signal and the filter. A procedure is suggested which consists of analyzing the same signal with different filter bandwidths and which enables one to distinguish between type I and type II signals in most cases. The validity of the theory is checked on computersimulated wave forms. Application is also made to natural magnetospheric emissions recorded both on the ground an on board satellites. It is shown that whereas the Pc 1 fine structure seems to be a real effect, there is no clear evidence that chorus VLF emissions do present a banded structure.