Research On Lock-in Amplifier Controlled By ARM

In recent years, the number of road accidents and fatalities overall showed anupward trend, while drunk driving is an important cause. Detecting whether there issuspicion of drunk driving quickly is of great importance. Using TDLAS technology(Tunable Diode Laser Absorption Spectroscopy) can detect trace gases quickly andaccurately. During the TDLAS gas detection technology, lock-in amplifier isavailable to extract the second harmonic of the detected signal, which is helpful forthe identification of the presence of alcohol gas.In order to face the need of the detection of alcohol gas, the paper designed alock-in amplifier controlled by ARM which is useful to extract the second harmonic.First of all, it introduced the principle of TDLAS technology an lock-in amplifier. Onthe base of the theory, I designed the hardware circuit. It contains the following parts:the signal preprocessing circuit, overload recognition circuit, phase shift circuit,phase-sensitive detection circuit, AD conversion circuit, FIR filter circuit and signalprocessing circuit. The lock-in amplifier uses ARM7microprocessor LPC2148as thecentral control unit for controlling the magnification, the phase shift angle, the filterparameters, etc. The design of the signal preprocessing circuit and thephase-sensitive detection circuit are completed by analog circuit while the phaseshift circuit and the FIR filter circuit are completed by using digital circuit. The dataof the filter is transferred to ARM in the form of digital signal. The experiment is divided into two parts: the lock-in amplifier testingexperiment and the alcohol gas detection experiment. In the lock-in amplifier testingexperiment, we used weak sinusoidal signal with strong noise as the input signal oflock-in amplifier and the signal is detected successfully. In the alcohol gas detectionexperiment, we connected the detector signal to the lock-in amplifier and determinewhether alcohol gas occurs by analyzing the output data of lock-in amplifier. Theresult shows that the lock-in amplifier can accurately extract the second harmonicsignal and can be applied to the field of TDLAS gas detection.