95 GHz silicon germanium low noise amplifier as front-end receiver for sparse aperture millimeter wave imaging
University of Delaware
Demand for the ability to navigate in degraded visual environments (DVE) such as dust, smoke, and fog, has lead the development of millimeter wave (mm-wave) real-time imaging systems. Millimeter wavelength radiation has shown that the wavelengths are long enough to penetrate the obscurants while also allowing sufficient resolution. A low attenuation atmospheric window in the 95 GHz region has pushed for these systems to operate at these millimeter wavelength frequencies. Due to low signal levels at these frequencies, the system requires high gain in the front-end to boost the signals of mm-wave frequencies. This involves collecting the electromagnetic waves with a horn antenna and then amplifying the signal with a low noise amplifier (LNA) to maximize the signal to noise ratio (SNR). In photonics-based imaging systems, the mm-wave signal is then up-converted to optical domain, where it then propagates through optical fibers to an infrared camera for further processing. The horn, LNA and up-converter comprise a single module. A large distributed array of modules, around 200, are required for a real-time mm-wave imaging system capable of peering through DVE. As a result, pushing this technology to higher frequencies can be very costly, due to the high prices of individual high frequency components. Therefore, an alternative technology is required to keep the costs to a minimum. One approach to controlling costs of components operating at higher frequencies is to adopt an alternative amplifier technology. Conventionally, commercially available GaAs and InP LNAs are used to obtain high gain at the high frequencies, but at 95 GHz, each amplifier used to be thousands of dollars. Since then, costs for each amplifier decreased to $100. With each module requiring three or more amplifiers, costs become prohibitively high for many applications. Therefore, this thesis focuses on the development of a 95 GHz amplifier using silicon germanium (SiGe) technology to obtain the required high gain while maintaining low costs. To date, extensive efforts have been made in the development of SiGe amplifier technology and high gain was demonstrated at the W-band. However, existing amplifier technology does not meet the requirements of the mm-wave imager. In particular, major limitation is the 3-dB bandwidth of the gain curve. A distributed aperture system with a wide field of view and broadband response will experience a phenomenon known as fringe-washing if an off-axis signal arrives with significant delay between the receivers on the opposite ends of the longest baseline. Severe fringe-washing occurs when this delay of the projected baseline is comparable to the correlation time of the signal, i.e. the inverse bandwidth of the system. To mitigate fringe-washing, each module must limit the bandwidth of operation, which can be accomplished either with a filter or an amplifier. Since filters can be lossy, ideally a narrow-band amplifier is preferred. In this thesis, using the basic principle of amplifier design, an LNA is developed based on advanced SiGe-foundry processes to operate in the 95 GHz regime. The advantage of a custom SiGe amplifier is the ability to design it to meet the imager's specific demands, including gain, noise figure, bandwidth, and power consumption in a single low cost device. This thesis details such design, and includes the discussion of tradeoffs and limitations imposed by the commercial SiGe-foundry processes employed.