Design and analysis of subharmonic mixer at 183ghz for remote sensing applications

Abstract

Sub-millimetre wave band holds a rich amount of information related to atmospheric molecules in form of spectral lines which originate due to vibration and rotation of various molecules. By analyzing the behavior of the spectral lines various conditions like temperature, humidity, wind and pressure can be found. Radiometers are used for extracting the relevant information from these spectral lines. newline newlineIn this thesis design of front-end receiver used for receiving such low intensity emissions from various molecules has been presented. Sub-Harmonic mixers which act as a front-end receiver for receiving very low intensity signals emitted from molecules. Sub-Harmonic receiver converts the sub-millimetre wave frequencies to IF frequencies where various measurements can be done for predicting the presence and other characteristics of molecules. newline newlineSub-Harmonic mixer presented utilizes a pair of diodes for eliminating even mode harmonics. Stub matching at RF and LO frequencies has been implemented to prevent the leakage of LO frequency at IF and RF ports similarly for reducing the leakage of RF at LO and IF ports. Waveguide to microstrip transition at RF and LO frequencies have been implemented for matching embedded impedance of diode. 3-D EM wave simulator HFSS (High Frequency Structure Simulator) software has been used for design of waveguide transitions. newlineDue to lack of measurement instruments at higher frequency, for proof of concept waveguide to microstrip transition has been fabricated for Ka-Band frequencies. Waveguide to microstrip transition has been simulated and tested on RT5880 substrate. Measured results show a good compromise with simulated results. Microstrip line low pass filter at IF port has been added to prevent any leakage of LO and RF frequencies at IF port. newline newlineEM simulation using ADS (Advanced Design Systems) has been carried for planar structure design like low pass filter, short circuit and open circuit stub along with diode model.