Monolithic Microwave Integrated Circuits for Wideband SAR System

In this thesis a technology study of monolithic microwave integrated circuits (MMIC’s) for future SAR systems operating at L-, C-, and X-band is performed. As a prerequisite for SAR applications, these MMIC’s should demonstrate wideband performance with a high degree of gain flatness and phase linearity. The wide signal bandwidth required in future SAR systems makes the development of MMIC’s for this application challenging. Integrated circuit technologies suitable for implementing the key components in a wideband SAR system are identified. These are a 0.8µm, 35 GHz f_T SiGe HBT process for the quadrature modulator/demodulator subsystem and a 0.2µm, 63 GHz f_T GaAs pHEMT process for the RF up/downconverter subsystem.

An investigation of the influence of substrate effects on the frequency response of SiGe HBT wideband MMIC’s is performed. As part of the investigation, parameter extraction methods for device models including substrate effects are developed for SiGe HBT devices, pad structures, interconnect lines, and poly resistors. A direct parameter extraction method suited for modern poly-silicon SiGe HBT devices is presented. The applicability of the direct parameter extraction method for VBIC95 model parameter extraction is successfully demonstrated.

Wideband active mixers and input buffers are key components for the quadrature modulator/demodulator subsystem. An analysis of the main bandwidth limitations in active mixers based on the Gilbert Cell mixer topology is performed. Advanced circuit techniques are exploited, useful when the objective is to achieve wideband operation with high degree of gain flatness and phase linearity. Experimental results for a wideband active mixer implemented in the SiGe HBT process achieves a 8.5 dB conversion gain with 3 dB bandwidth of 11 GHz and 7.5 GHz for the input port and output ports respectively. The experimental results are well predicted by simulations and demonstrates state-of-the-art results compared with other wideband active mixers implemented in comparable processes. The experimental results for an input buffer in the SiGe HBT process achieves a 3 dB bandwidth of 6.6 GHz with excellent phase linearity. These experimental results are well predicted by simulations. The developed key components are well suited for further integration into quadrature modulator/demodulator SiGe HBT MMIC’s.

Key components for the RF up/downconverter includes wideband active mixers, active baluns and active output combiners. A fully integrated RF up/downconverter including active mixer, active baluns on the input and LO port, and an active output combiner on the output port has been implemented in the GaAs pHEMT process. The experimental results for the GaAs pHEMT MMIC achieves a 10 dB conversion gain and a 9.5 GHz 3 dB input port bandwidth. The results are comparable to best published results for state-of-the-art highly integrated wideband active mixers based on FET technologies. RF downconversion from C-band to L-band is demonstrated with an excellent conversion gain flatness over an 800 MHz bandwidth.