Development Of Digital Demodulation Signal Acquisition System For Portable Near-infrared Spectrometer

The near-infrared spectrometer are used as carrier of the near-infraredspectroscopy, which offered a lot of advantages as non-destructive sample processing;without chemicals, green non-polluting; in-field fast and real-time on-lineanalysis. Signal acquisition system as an important part of the near-infraredspectrometer, directly affects the spectrometer’s working performance, its designproject affects miniaturization of the spectrometer. It is necessary to research on thekey technologies of the signal acquisition system.NIR Laboratory of Jilin University has successfully developed the gratingscanning portable near-infrared spectrometer PISA-S. Modulation of the PISA-S isthe optical chopper, which belongs to the external modulation. By controlling DCmotor to drive sectored reticle rotation, light from halogen lamp will be modulated asa specific frequency alternating light. In PISA-S, conditioning and demodulation areimplemented to the analog signal by using lock-in amplifier (LIA). Phase sensitivedetector (PSD) is the core component of LIA, modulation is implementedto modulation signal by using it.By using the4N times of signal frequency sampling digital demodulationmethod, conditioning and demodulation are implemented to the signal. Comparedwith the phase-sensitive detection method of PISA-S, this method has simple andflexible features of digital signal processing, spurned complex analog signalprocessing circuits, reduced the debugging steps, greatly retrenched the circuit of thespectrometer.Altera Corporation’s product EPM240T100C5N, which has enough I/O ports forthe system expansion and used to control ADC and motor, is applied in this paper;ADS8505is selected as signal acquisition ADC; STC89LE54RD+is selected asmicrocontroller. Timing pulse generated by CPLD controls the ADC to collect data,data converted electrical level by SN74LVTH162245and sent to the CPLD., the datasent to the microcontroller after pretreatment in CPLD, the final data processing iscompleted in microcontroller and uploaded to the PC through the serial port.After the hardware design is finished, we use the Quartus II development environment to design the CPLD internal program. The program includes threemodules: a motor drive, the data acquisition and data pretreatment. By modifying theprogram of the microcontroller according to digital demodulation, the program ismatched with the upper software by MATLAB.After the whole system design is completed by the new demodulation scheme, aprototype model PISA-S-4N is successfully assembled. Compared with the originalprototype, three experiments are designed: the Influence of number of signalperiodsN Tvalue to S/N, the Influence of sampling frequency to S/N,frequency-doubled different signal control A/D sampling and the spectral datacomparison test between PISA-S-4N and PISA-S, then draw the correspondingconclusion.Finally, the general content of the paper are summarized. Some aspects of thesystem still need to be improved.