Multicarrier reflectometry device for on-line diagnosis of at least one transmission line, characterised in that it comprises: • a transmission part connected to this transmission line, which comprises successively: - a module for parameterising a test signal S F, such that: s F = 0, c 1 e j θ 1, c 2 e j θ 2,..., C N / 2 − 1 e j θ N / 2 − 1, c N / 2, C N / 2 − 1 e j θ N / 2 − 1, c N / 2 − 2 e j θ N / 2 − 2,..., c 1 e j θ 1 T where N corresponds to the number of sinusoids, c n ∈ R + is the amplitude of the n th sinusoid, θ n is the phase of the n th sinusoid, - a module for synthesising this test signal by inverse discrete Fourier transform, - a digital to analogue converter, connected to this transmission line, which delivers to the latter an analogue test signal x(t) that is a periodic signal, • a detection part, connected to this transmission line, which comprises successively: - an analogue to digital converter, connected to this transmission line, which receives a measured signal x s (t) and digitises it, - a discrete Fourier transform module, - an analysis module containing a deconvolution module, which performs a post-processing of the measured signal thus transformed using the test signal S F and which makes it possible to estimate the transmission response h(t), such that x s (t)=x(t)*h(t), * representing a convolution product. • a transmission part connected to this transmission line, which comprises successively: - a module for parameterising a test signal S F, such that: s F = 0, c 1 e j θ 1, c 2 e j θ 2,..., C N / 2 − 1 e j θ N / 2 − 1, c N / 2, C N / 2 − 1 e j θ N / 2 − 1, c N / 2 − 2 e j θ N / 2 − 2,..., c 1 e j θ 1 T where N corresponds to the number of sinusoids, c n ∈ R + is the amplitude of the n th sinusoid, θ n is the phase of the n th sinusoid, - a module for synthesising this test signal by inverse discrete Fourier transform, - a digital to analogue converter, connected to this transmission line, which delivers to the latter an analogue test signal x(t) that is a periodic signal, - a module for parameterising a test signal S F, such that: s F = 0, c 1 e j θ 1, c 2 e j θ 2,..., C N / 2 − 1 e j θ N / 2 − 1, c N / 2, C N / 2 − 1 e j θ N / 2 − 1, c N / 2 − 2 e j θ N / 2 − 2,..., c 1 e j θ 1 T where N corresponds to the number of sinusoids, c n ∈ R + is the amplitude of the n th sinusoid, θ n is the phase of the n th sinusoid, where N corresponds to the number of sinusoids, c n ∈ R + is the amplitude of the n th sinusoid, θ n is the phase of the n th sinusoid, - a module for synthesising this test signal by inverse discrete Fourier transform, - a digital to analogue converter, connected to this transmission line, which delivers to the latter an analogue test signal x(t) that is a periodic signal, • a detection part, connected to this transmission line, which comprises successively: - an analogue to digital converter, connected to this transmission line, which receives a measured signal x s (t) and digitises it, - a discrete Fourier transform module, - an analysis module containing a deconvolution module, which performs a post-processing of the measured signal thus transformed using the test signal S F and which makes it possible to estimate the transmission response h(t), such that x s (t)=x(t)*h(t), * representing a convolution product. - an analogue to digital converter, connected to this transmission line, which receives a measured signal x s (t) and digitises it, - a discrete Fourier transform module, - an analysis module containing a deconvolution module, which performs a post-processing of the measured signal thus transformed using the test signal S F and which makes it possible to estimate the transmission response h(t), such that x s (t)=x(t)*h(t), * representing a convolution product.