Circuit for detecting the envelope of a signal modulated in a high-frequency band, i.e. in a frequency band located above 30 GHz, this circuit comprising: - an input port (P e34) able to receive the signal modulated in the high-frequency band, this modulated signal comprising an envelope modulated by a carrier at a preset frequency f c comprised in the high-frequency band, the envelope comprising a constant ν 1 added to a useful signal s fi (t), most of the power of the useful signal sfi(t) being comprised inside a useful frequency band centred on an intermediate frequency f i and of width L u, - an output port (P s34) via which the detected envelope of the modulated signal is delivered, - a first multiplier connected to the input port and able: • to multiply a first example of the signal received on the input port by itself, and • to deliver on an output this first example of the received signal multiplied by itself, - a modifier connected, in parallel to the first multiplier, to the input port, this modifier being able: • to modify the amplitude of the power spectrum, of a second example of the signal received on the input port, at the frequency f c without modifying the amplitude of this power spectrum in the useful frequency band, and • to deliver on an output the modified signal, - a second multiplier connected to the output of the modifier and able: • to multiply the modified signal by itself, and • to deliver on an output the modified signal multiplied by itself, - a subtractor comprising first and second inputs connected to the outputs of the first and second multipliers, respectively, this subtractor being able: • to subtract the signal received on its first input from the signal received on its second input, and • to deliver on an output the result of this subtraction, - a filter able to remove frequency components higher than or equal to 2f c in a signal obtained from the signal delivered by the output of the subtractor, this filter comprising to this end a -3 dB cut-off frequency comprised between f i +L u /2 and 2f c, this filter being able to deliver the result of this filtering, by way of detected envelope of the modulated signal, on an output connected to the output port of the envelope-detecting circuit. - an input port (P e34) able to receive the signal modulated in the high-frequency band, this modulated signal comprising an envelope modulated by a carrier at a preset frequency f c comprised in the high-frequency band, the envelope comprising a constant ν 1 added to a useful signal s fi (t), most of the power of the useful signal sfi(t) being comprised inside a useful frequency band centred on an intermediate frequency f i and of width L u, - an output port (P s34) via which the detected envelope of the modulated signal is delivered, - a first multiplier connected to the input port and able: • to multiply a first example of the signal received on the input port by itself, and • to deliver on an output this first example of the received signal multiplied by itself, • to multiply a first example of the signal received on the input port by itself, and • to deliver on an output this first example of the received signal multiplied by itself, - a modifier connected, in parallel to the first multiplier, to the input port, this modifier being able: • to modify the amplitude of the power spectrum, of a second example of the signal received on the input port, at the frequency f c without modifying the amplitude of this power spectrum in the useful frequency band, and • to deliver on an output the modified signal, • to modify the amplitude of the power spectrum, of a second example of the signal received on the input port, at the frequency f c without modifying the amplitude of this power spectrum in the useful frequency band, and • to deliver on an output the modified signal, - a second multiplier connected to the output of the modifier and able: • to multiply the modified signal by itself, and • to deliver on an output the modified signal multiplied by itself, • to multiply the modified signal by itself, and • to deliver on an output the modified signal multiplied by itself, - a subtractor comprising first and second inputs connected to the outputs of the first and second multipliers, respectively, this subtractor being able: • to subtract the signal received on its first input from the signal received on its second input, and • to deliver on an output the result of this subtraction, • to subtract the signal received on its first input from the signal received on its second input, and • to deliver on an output the result of this subtraction, - a filter able to remove frequency components higher than or equal to 2f c in a signal obtained from the signal delivered by the output of the subtractor, this filter comprising to this end a -3 dB cut-off frequency comprised between f i +L u /2 and 2f c, this filter being able to deliver the result of this filtering, by way of detected envelope of the modulated signal, on an output connected to the output port of the envelope-detecting circuit.