Abstract: As a combination of traditional swept spectrum analyzer and Fast Fourier Transform technology, vector signal analyzer can digitally process signals, analyze various modulated signals, and obtain the amplitude and phase information of the measured signals simultaneously, which is of great significance for modern communications. With the application frequency of electromagnetic wave expanding to millimeter-wave and even Terahertz band, the measurement frequency of vector signal analyzer is also urgently needed to expand to a higher frequency band, which can be achieved by installing a downconverter in the front end of the system. This paper carries out research based on the down-conversion technology of 50 GHz~110 GHz broadband signals, and overcomes the key devices in the frequency conversion system, including the wideband switch for time-division 50 GHz~110 GHz signals, the wide stopband waveguide filter for suppressing mirror frequency signals, and the local oscillator component based on the frequency doubling amplifier module. At the same time, the 50 GHz~110 GHz broadband signal down-conversion system is built, and the 50 GHz~110 GHz broadband signal is down-converted to 4.6 GHz~39.6 GHz. The experiments show that the flatness of the down-converted signal is less than ±3.5 dB in any 2 GHz, the suppression of the mirror frequency signal is greater than 60 dBc, the output stray suppression is greater than 40 dBc, and the LO leakage suppression is greater than 70 dBc, which meets the requirements of the system index.