Low power rf transceivers.
PhD thesis, University of Glasgow.
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This thesis details the analysis and design of ultra-low power radio transceivers operating at microwave frequencies. Hybrid prototypes and Monolithic Microwave Integrated Circuits (MMICs) which achieve power consumptions of less than 1 mW and theoretical operating ranges of over 10 m are described.
The motivation behind the design of circuits exhibiting ultra low power consumption and, in the case of the MMICs, small size is the emerging technology of Wireless Sensor Networks (WSN). WSNs consist of spatially distributed ‘nodes’ or ‘specks’ each with their own renewable energy source, one or more sensors, limited memory, processing capability and radio or optical link. The idea is that specks within a ‘speckzone’ cooperate and share computational resources to perform complex tasks such as monitoring fire hazards, radiation levels or for motion tracking. The radio section must be ultra low power e.g. sub 1 mW in order not to drain the limited battery capacity. The radio must also be small in size e.g. less than 5 x 5 mm so that the overall speck size is small. Also, the radio must still be able to operate over a range of at least a metre so as to allow radio contact between, for example, rooms or relatively distant specks.
The unsuitability of conventional homodyne topologies to WSNs is discussed and more efficient methods of modulation (On-Off Keying) and demodulation (non-coherent) are presented. Furthermore, it is shown how Super-Regenerative Receivers (SRR) can be used to achieve relatively large output voltages for small input powers. This is important because baseband Op-Amps connected at the RF receiver output generally cannot amplify small signals at the input without the output being saturated in noise (10mV is the smallest measured input for 741 Op-Amp). Instrumentation amplifiers are used in this work as they can amplify signals below 1mV.
The thesis details the analysis and design of basic RF building blocks: amplifiers, oscillators, switches and detectors. It also details how the circuits can be put together to make transceivers as well as describing various strategies to lower power consumption. In addition, novel techniques in both circuit and system design are presented which allow the power consumption of the radio to be reduced by as much as 97% whilst still retaining adequate performance. These techniques are based on duty cycling the transmitter and receiver and are possible because of the discontinuous nature of the On-Off Keying signal. In order to ease the sensitivity requirements of the baseband receive amplifier a design methodology for large output voltage receivers is presented. The designed receiver is measured to give a 5 mV output for an input power of -90 dBm and yet consumes less than 0.7 mW.
There is also an appendix on the non linear modelling of the Glasgow University 50nm InP meta-morphic High Electron Mobility Transistor (50nm mHEMT) and one on the non linear modelling of a commercial Step Recovery diode (SRD). Models for the 50 nm mHEMT and the SRD are useful in the analysis, simulation and design of oscillators and pulse generators respectively.
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