Abstract: An apparatus for analyzing a spectrum of an input signal (?(t)) having at least one line with a center frequency ({circumflex over (?)}x) at the center of the line includes: a mixer for zero-mixing the input signal (?(t)) to produce a base band signal (z(t)) by sweeping a local oscillator frequency (?s) generated by a local oscillator, a resolution filter for filtering the base band signal (z(t)) to produce a filtered base band signal (y(t) ), and an envelope reconstruction means for reconstructing the envelope (E(?)) of the spectrum of the input signal (?(t)) by using an estimated amplitude (?x) at an estimated center frequency ({circumflex over (?)}x) of each line of the input signal (?(t)). Further, only the real component (I) of the base band signal (z(t)) is filtered with the resolution filter and the imaginary component (Q) is generated from the filtered base band signal (y(t)) by performing a Hilbert transform in a Hilbert filter arranged downstream of the resolution filter.

Abstract: An apparatus (1) for analyzing a spectrum of an input signal (x(t)) having at least one line with a center frequency (?x) at the center of the line comprises: a mixer (2) for zero-mixing the input signal (x(t)) to produce a base band signal (z(t)) by sweeping a local oscillator frequency (?s) generated by a local oscillator (3), a resolution filter (4) for filtering the base band signal (z(t)) to produce a filtered base band signal (y(t)) and envelope reconstruction means (27) for reconstructing the envelope (E(?)) of the spectrum of the input signal (x(t)) by using an estimated amplitude (?x) at an estimated center frequency ({circumflex over (?)}x) of each line of the input signal (x(t)). Only the reel component (I) of the base band signal (z(t)) is filtered with the resolution filter (4) and the imaginary component (Q) is generated from the filtered base band signal (y(t) by performing a Hilbert transform in a Hilbert filter (21) arranged downstream of the resolution filter (4).