Sideband generation for submillimeter wave applications

Sideband generation is a method for producing tunable sources in the far infrared frequency range by mixing a tunable microwave source with a fixed laser source to produce tunable sidebands. Schottky mixer diodes have been used in comer cube mounts for sideband generation, but have very poor conversion loss (31 dB) and output power (10 muW). This thesis examines the differences between an upconverter and a downconverter, then reports a 17 dB improvement in the conversion loss of the state-of-the-art sideband generation by using a varactor in a waveguide that is optimized specifically for sideband generation. Also, quasi-optical methods of power combining using Schottky mixers are demonstrated that if used with varactors have the potential for an improvement of three orders of magnitude in output power over traditional comer cube mixers.;These results represent the first use of a varactor for sideband generation in THz frequencies. So, first an 80 GHz proof-of-principle varactor sideband generator was fabricated and tested to develop the circuit models for scaling to 1.6 THz. Circuit diagnostic equipment such as the HP8510 network analyzer are available at these frequencies and provided the measurements necessary to understand and characterize the circuit. High frequency effects such as plasma resonance need to be considered with varactors at 1.6 THz. The circuit model developed at 80 GHz was incorporated with a high frequency diode model in order to design the varactor. The varactors have 10 times more anode area than the best mixer diodes, 7.3 V breakdown voltage, and 17 dB improvement in conversion loss. Implemented in reduced height waveguide with a planar whisker and a tunable backshort, the varactor sideband generator resulted in 14 dB SSB conversion loss with 55 muW output power that was only limited by the available input power. The improvements result from increased phase modulation by the varactor and also more optimal embedding impedances provided by the waveguide environment. The effects of just the embedding impedances and coupling are also presented by measuring the performance of a UVA-NF1T2 mixer diode in the same waveguide block. The conversion loss of 24 dB represents a significant improvement over similar diodes in comer cube mounts, but performed much worse than a varactor in the same environment. Both devices were also tested as mixers. The UVA-NF1T2 mixer diode resulted in an IF corrected noise temperature of 7800 K. The varactor in the same environment performed better as a sideband generator than as a mixer as predicted by the simulations. There was a strong correlation between the measured results and simulation which provide a good base to develop more rugged systems that either use planar varactors in micromachined blocks or planar varactor arrays.