Study On Large Dynamic Range And High Speed Photon Correlator

Photon Correlation Spectroscopy (PCS) provides fast and accurate particle size on suspended particles in the nanometer size range. Numerous applications have been reported in different scientific and technological areas. The photon correlator is the key equipment of photon correlation instruments which counts the photon pulse and computes intensity autocorrelation function real-time. The correlators we used are almost depend on imported products. Therefore, designing and developing the correlator with independent intellectual property rights will bring huge economy and social benefits.The paper based on expand correlator’s dynamic range and improve the correlator’s operation speed together with improve the normalization accuracy of the autocorrelation function. The project consists of five parts:(1) The structures and characteristics of linear correalator, ratio correlator and Multi-tau correlator based on PCS theory were studied(2) The FPGA was used for designing double-counter which can count the photon pulse ceaseless. The mass memory of DSP was used for designing the ring shift register which realized large dynamic range ratio photon correlator. The data was transferred between personal computer and DSP with USB interface. We proposed the channel lag time calculation formula of ratio photon correlator and analyzed the normalization of autocorrelation function.(3) The operating principle of Multi-tau correlator was studied. The FPGA was used for designing and achieving Multi-tau correlator. The key module is correlation operation module which consists of shift register, multiplier, accumulator and data converter, every part was developed in Verilog language. Triangular averaging and normalization of autocorrelation function was analyzed.(4) The large dynamic range high speed correlator was designed with DSP and FPGA. The ratio photon correlator based DSP expand the dynamic range and Multi-tau correlator based on FPGA improve the operation speed.(5) The correlator which was designed above constitutes the PCS particle sizing system. We measured60nm,150nm,200nm,300nm and450nm standard polystyrene particles at25℃and the results showed that the large dynamic range high speed correlator was designed in the paper can improve the accuracy of particle size measurement. It can make the relative error less than5%and repeatability error less than3%only if choose appropriate sample time and dynamic range. We studied the PCS theory, photon correlator structure and normalization of autocorrelation function and designed the high speed Multi-tau photon correlator with DSP and FPGA which can achieve100ns sample time and1010dynamic range.