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A new class of time-varying lowpass and highpass filters is described in which the stopband attenuation is obtained by outphasing in a quadrature-modulation arrangement, the undesired associated frequency translation or inversion being cancelled by a suitably chosen second step of quadrature demodulation or both; bandpass and bandstop filters can be obtained as combinations of lowpass and highpass filters.
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W. Saraga (1967) studied this question.
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