Sammendrag
The real and imaginary parts of base band signals obtained from a real narrow band signal are obtained by quadrature mixing, namely by using cosine and sine mixing signals at the narrow band's center frequency. We address the consequences of errors in the relative phase of the outputs of the quadrature mixer, in particular, the case when the phase error is introduced when digital samples of the quadrature base band signals are not synchronised, e.g., when the imaginary (real) component has been shifted by one sample with respect to the real (imaginary) component. The main consequence of this type of error is in making the spectrum of the signal increasingly more symmetric as a function of frequency. Thus, if the true spectrum is asymmetric --- for instance, owing to a Doppler shift of an originally symmetric spectrum --- then erroneous symmetric frequency components will be introduced. Here we attempt to asses the consequences of such error in some EISCAT experiments. In particular, one such experiment when the University of Tromsø's (UiTø) software radar gathered raw data from the same receiver channel as the faulty EISCAT system permits to compare uncontaminated and contaminated data. The ionospheric signals as measured by the uncontaminated UiTø system show unusually large Doppler shifts increasing with height (accelerated plasma flow) while the EISCAT system shows mostly symmetric spectra widened by the symmetrisation effect as expected. The faulty spectra were reproduced from the UiTø system by delaying the imaginary samples by one sample period. In the general case, normal incoherent scatter signals with no Doppler shift are not affected by the error because they are symmetric to start with. However, plasma parameters obtained by fitting the faulty data will be increasingly erroneous as the Doppler shift of the true signal increases while the fitted velocity will be artificially small under all practical circumstances.
Vis fullstendig beskrivelse
