Measurements of the Nonlinearity of the Ultra Wideband Signals Transformation

Abstract

The linearity is one of the more difficult challenges of receiver in ultra wideband (UWB) communication systems (Green & Roy, 2003). When testing UWB receivers, one should use UWB signals as nonlinear signal distortion caused by a device dependant on the waveform of a signal. The investigation of nonlinear distortions of UWB signals run across considerable difficulties. They are caused by a continuous spectrum of UWB signals. In this case, it is impossible to observe harmonics or intermodulation products. In addition, application of UWB signals practically has no alternative in subsurface radars. However, such radars remain linear today. It can be explained by the same reason as stated above (difficulties in observing nonlinear transformation products). The same situation can be observed in reflectometry of wire transmission lines. Lately Agilent Technologies Company has been using X-parameters (Verspecht, 1996; Verspecht & Root, 2006) in Advanced Design System (ADS) and PNA-X measuring devices. It is assumed that object characteristics depend only on the first harmonic of test signal and dc bias. Therefore, X-parameters are adequate only when narrow-band test signals are used. The methods described, which allow using the UWB test signals, have some failings. There is a method, which allows identifying parameters of nonlinear object model by means of testing the object by pulse signal with level sweep (Sobhy et al., 1996). However, such model includes recursive (or nonrecursive) filter and the order of this filter is prespecified. Therefore, if complexity of the object transfer function is not limited, the method is not suitable. The equivalent gain concept (Arnstein, 1979; Arnstein et al., 1992; Chen et al., 1996) implies finding the difference between the object response and the test signal. In this case it is required that the effective width of the test-signal spectrum should be inside the horizontal segment of the frequency response of the object under test. Otherwise, it is necessary to compensate linear distortions of the test signal produced by the object. In practice, this compensation can be accomplished only for time-independent linear distortions with simple frequency dependence. The problem of observing nonlinear transformation products of UWB signals can be solved by using the test signal with local null (or nulls) of spectrum (E. Semyonov, 2002, 2004; Lipshitz et al., 2002) or by means of rejection of narrow frequency band in the test-signal