In this paper, a digital switching demodulator is presented for use in ac-based electrical capacitance tomography systems. Implementing a switching phase-sensitive demodulator (PSD) digitally offers the following advantages: 1) Demodulation can be implemented using a programmable digital device, and hence, CMOS switches, which are used in a conventional switching PSD, are no longer needed; 2) compared with the widely used digital quadrature PSD, this proposed demodulator is simple in configuration because neither a reference signal nor multiplication is required; 3) according to the specific requirements, the new demodulator can be implemented in two operation modes, i.e., the amplitude mode and the phase-sensitive mode; and 4) because only subtractions and accumulations are needed, the proposed demodulator can be easily implemented with low-cost logic devices, e.g., a complex programmable logic device (CPLD). By simulation, the feasibility and effectiveness of the proposed demodulator have been confirmed. CPLD-based and field-programmable-gate-array-based capacitance measurement circuits are constructed, and the performances of different demodulation methods are compared. Both simulation and experiment show that the proposed demodulator can provide demodulation results with high signal-to-noise ratio. The system design can be simplified using the digital switching demodulator.
['Lijun Xu', 'Haili Zhou', 'Zhang Cao', 'Wuqiang Yang']
A Digital Switching Demodulator for Electrical Capacitance Tomography