Research On Frequency Error And Digital Demodulation Methods For Modulated Signal In Near Infrared Spectrometer

Portable Near-infrared(NIR)spectrometer offers rapid,nondestructive and in situ analysis of the molecular structure and composition.Scanning portable NIR spectrometer has the characteristics of wide spectral range,simple structure and high cost performance.However,the frequency error of modulated signal influences the accuracy of the measured amplitude.Thus,the research of frequency error and digital demodulation methods for modulated signal in scanning portable NIR spectrometer has an important significance for the promotion of international competitiveness of domestic instrument.The paper studied frequency error of the modulated signal in scanning portable NIR spectrometer,established a mathematical model of the modulated amplitude and frequency error in the digital lock-in amplifier,proposed three digital demodulation algorithms for the modulated signal,designed a digital demodulation circuit system,and the experimental research on the application of three algorithms in spectrometer.The main researches are as follows.1.A functional relationship between demodulated amplitude and frequency error was established.When the measured signal frequency had system error or random error,the demodulated amplitude also had corresponding system error or random error,and was related to the phase of the measured signal.2.Three digital demodulation algorithms were proposed.(1)Digital demodulation algorithm based on frequency multiplication sampling at zero-phase.By setting the phase of sampling start point to zero,the demodulated amplitude random error caused by phase change was reduced when there was frequency system error.(2)Digital demodulation algorithm based on numerical integration.The phase sensitive detection and low pass filtering in digital lock-in amplifier were replaced by the numerical integration and average operation in the complete period.The algorithm was adaptive the frequency fluctuation.The number of sampling points in each period was adjusted with the change of the measured signal frequency,which reducing random error of the demodulated amplitude caused by frequency random error.(3)Digital demodulation algorithm based on frequency correction.The ratio of sampling frequency to modulated signal frequency was employed to replace the number of sampling points in the amplitude demodulation calculation.The rounding error caused by the frequency fluctuation of the measured signal was corrected,improving the frequency adaptability of the amplitude demodulation algorithm.3.A digital demodulation circuit system was designed.The circuit system employed digital signal processor(DSP)to demodulate the modulation signal converted by the high speed ADC,which had the function of zero phase judgment and frequency measurement.The sine sweep signal from signal generator was detected by three digital demodulation algorithms.The signal-to-noise ratio of the demodulation results were 51.03 d B,59.05 d B and 90.16 d B,respectively.4.Three digital demodulation algorithms were applied to scanning portable NIR spectrometer.The signal-to-noise ratio of the spectral reflectance was 53.49 d B,52.59 d B and 55.52 d B,respectively.In order to reduce the circuit noise,the digital demodulation circuit was simplified to suit the digital demodulation algorithm based on numerical integration and frequency correction.The signal-to-noise ratio of the spectral reflectance in the simplified system was 60.69 d B and 65.53 d B,respectively,which was higher than the SNR(58.01 d B)of the analog demodulation system.The research results showed that the digital demodulation algorithm based on frequency correction was suitable for the detection of modulated signal.The signal to noise ratio of the spectral reflectance reached 65.53 d B.The demodulation accuracy of the modulated signal in the scanning portable NIR spectrometer was improved.