OSCILLATOR-TYPE CAPACITIVE SENSOR INTERFACING CIRCUITS

Abstract

(I) Problem Description (a) Modulator (Circuit) Relaxation oscillators can be used as modulators in capacitive interface circuits. Their instantaneous frequency of oscillations is usually not equal to its theoretical value. Hence compensation methods are proposed and frequency of oscillations is derived. Further, use of a relaxation oscillator as a modulator requires a scheme of modulation which relates the unknown capacitance to some characteristic of the oscillations generated by it. (b) Demodulator (System) Once the modulated signal from relaxation oscillator is obtained, it’s needed to be demodulated using a suitable demodulator. Here, a system level approach can also be used to devise a scheme of demodulation using the tools of DSP (Digital Signal Processing). SDFT (Sliding Discrete Fourier Transform) is one such use ful tool which can be used and a method of demodulation can be proposed based around SDFT as the main error generating mechanism for detection of instanta neous changes in the incoming modulated signal. (II) Motivation The main motivation is to design a complete capacitive sensor. Capacitive sensors are perhaps one of the most widely used types of sensor systems. This sensor focuses primarily on the capacitive measurements in the range of pF-nF. (III) Work done The work is done on both circuit and system level. Circuit level work includes anal yses, derivation and verification of frequency of oscillations. The main work in this area is done in a circuit called Compensated Relaxation Oscillator which is finally used as a modulator. It’s analyzed and an equation is derived from which its fre quency of oscillations can be obtained. All oscillators are studied and simulated. A novel scheme of modulation is proposed based on the output of relaxation oscil lator. This modulated output can be interpreted as a frequency modulated wave iii with a square wave carrier. The demodulator which uses PLL (Phase Locked Loop) with SDFT lying at its heart is designed in MATLAB Simulink and its working is validated by simulation. (IV) Conclusion The methods proposed for compensation improve performance of oscillators signifi cantly. The scheme of modulation and demodulator system works successfully.

(I) Problem Description (a) Modulator (Circuit) Relaxation oscillators can be used as modulators in capacitive interface circuits. Their instantaneous frequency of oscillations is usually not equal to its theoretical value. Hence compensation methods are proposed and frequency of oscillations is derived. Further, use of a relaxation oscillator as a modulator requires a scheme of modulation which relates the unknown capacitance to some characteristic of the oscillations generated by it. (b) Demodulator (System) Once the modulated signal from relaxation oscillator is obtained, it’s needed to be demodulated using a suitable demodulator. Here, a system level approach can also be used to devise a scheme of demodulation using the tools of DSP (Digital Signal Processing). SDFT (Sliding Discrete Fourier Transform) is one such use ful tool which can be used and a method of demodulation can be proposed based around SDFT as the main error generating mechanism for detection of instanta neous changes in the incoming modulated signal. (II) Motivation The main motivation is to design a complete capacitive sensor. Capacitive sensors are perhaps one of the most widely used types of sensor systems. This sensor focuses primarily on the capacitive measurements in the range of pF-nF. (III) Work done The work is done on both circuit and system level. Circuit level work includes anal yses, derivation and verification of frequency of oscillations. The main work in this area is done in a circuit called Compensated Relaxation Oscillator which is finally used as a modulator. It’s analyzed and an equation is derived from which its fre quency of oscillations can be obtained. All oscillators are studied and simulated. A novel scheme of modulation is proposed based on the output of relaxation oscil lator. This modulated output can be interpreted as a frequency modulated wave iii with a square wave carrier. The demodulator which uses PLL (Phase Locked Loop) with SDFT lying at its heart is designed in MATLAB Simulink and its working is validated by simulation. (IV) Conclusion The methods proposed for compensation improve performance of oscillators signifi cantly. The scheme of modulation and demodulator system works successfully.