In order to construct a high-resolution terahertz spectrometer based on free electron lasers

a 4 THz eighth harmonic mixer was designed.The mixer adopts a hybrid transmission line structure

which reduces the difficulty of passive circuit design and processing

and at the same time ensures the isolation between the three ports of the mixer.The waveguide-suspension microstrip transition circuit model

filter model and Schottky diode model are designed and optimized in HFSS.Split the overall mixer circuit into three unit circuits

simulate and optimize the performance of each unit circuit in HFSS

and then splice it into a complete mixer for joint simulation optimization.Finally

the simulation results show that

driven by the local oscillator power of 5 dBm

in the frequency range of 3.4～4.4 THz

the single-sideband conversion loss of the mixer is better than 64.3 dB

and the RF return loss is better than 11 dB

in range of 3.4～4.0 THz

the mixer conversion loss is prior to 60 dB

the in-band flatness is better

and the overall circuit performance is relatively stable.