Advanced Schottky Diode Receiver Front-Ends for Terahertz Applications
Abstract: This thesis treats the development of high frequency circuits for increased functionality of terahertz receiver front-ends based on room temperature Schottky diode technology. This includes the study of novel circuit integration schemes, packaging concepts as well as new measurement and characterisation techniques. As the main result, a novel broadband waveguide integrated sideband separating (2SB) receiver topology for future Earth observation submillimetre wave instruments is proposed. The 2SB receiver topology has an inherent low RF and LO port voltage standing wave ratio (VSWR) and high sideband ratio (SBR). It is based on subharmonic (x2) Schottky diode double sideband (DSB) mixers with embedded IF low noise amplifiers (LNA's) and LO and RF 90 degree waveguide hybrids. Access to the IF IQ-paths makes it possible to implement phase and amplitude imbalance compensation schemes. Sideband separation is done in the analog domain by the use of an IF 90 degree hybrid or in the digital domain by using an IQ-correlator spectrometer. The use of embedded LNA's reduces the IF losses and leads to a low ripple and broadband response. Measured results on a prototype 2SB receiver operating in the 320 GHz to 360 GHz frequency range show an untuned SBR of 15 dB over the whole band and mixer noise consistent with the optimal performance of a DSB mixer. The LO return loss is measured to be approximately 15 dB (broadband) and the RF return loss is estimated to have similar performance. A 340 GHz DSB receiver with an embedded custom designed 3-15 GHz LNA has also been developed. By co-simulation of the mixer and LNA using a simple mixer noise model it is shown that accurate prediction of the receiver noise response is possible. The DSB receiver exhibits ultra low noise over the 12 GHz IF bandwidth, with a minimum input receiver noise temperature of 870 K (DSB). Two novel differential line phase shifters based on stepped impedance and coupled-line filter structures are proposed. The filters have a minimum lateral distribution making them well suited for use in submillimetre wave circuits. A method for TRL-calibration of terahertz monolithic integrated circuits (TMIC's) is also proposed and demonstrated. The method allows for embedded S-parameter characterisation of waveguide integrated TMIC devices and circuits.
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