Study of Signal Postprocessing Method of SigmaDelta Modulator for VoIP 

Author  PuShouJie 
Tutor  GuXiaoFeng 
School  Jiangnan University 
Course  Microelectronics and Solid State Electronics 
Keywords  Sigma  Delta modulator Quantization noise Cascaded integrator comb filter Halfband filter Compensation filter 
CLC  TN916.2 
Type  Master's thesis 
Year  2009 
Downloads  48 
Quotes  0 
SigmaDelta analogtodigital converter (ADC) is mainly composed by the SigmaDelta modulator and filter, compared to traditional ADC, its main advantage is that does not require complex analog circuit structure, its cost may be declining, while the characteristics of the digitized so that can be integrated into other digital chips, therefore, SigmaDelta ADC has been widely applied in the field of digital signal processing. Firstly, the input bandwidth of 20 KHz for VoIP chip 14 thirdorder SigmaDelta modulator. Selection of the the 111 cascade structure design modulator, model in MATLAB modulator. Simulation integration, discussed and adjusted coefficient of each module to determine the output of the integrator is not overloaded. Processing the output of the thirdorder SigmaDelta modulator levels. Derived through transform discrete signal Z domain, at all levels of the expression of the output quantization noise canceling circuit, merging the levels output behavior level simulation of the entire system. The results show that the signaltonoise ratio greater than 86dB, and an accuracy of 14, basically meet the design requirements. Subsequently, the postprocessing method on the the SigmaDleta modulator output signal. The structure of the the SigmaDleta modulator similar to the dualslope ADC, including an integrator and a comparator, and a feedback loop of a digitaltoanalog converter. This builtin digitaltoanalog converter is just a switch, it will the integrator input switching to a positive or negative reference voltage. SigmaDelta modulator to the high sampling frequency of sampling the input analog signal, and the difference between the two sampled low quantization, to thereby obtain the digital signal, and represented by a low bit digital SigmaDelta code. The code is then sent to a digital decimation filter decimation filtering, highresolution linear pulse code modulation digital signal. The decimation filter is actually equivalent to a pattern converter. The signal fed to the SigmaDelta ADC after the quantification of very low resolution (1), but the sampling frequency is very high. After digital filtering process, such oversampling is reduced to a relatively low sampling rate, while the resolution of the ADC is improved. Design a multilevel downsampling filter to reduce the highfrequency signal output by the SigmaDelta modulator. Analyzes a frequency characteristic of the cascaded integrator (CIC) filter, select the appropriate structure and order, to achieve a sampling frequency of the input signal decreased to 16 times, the attenuation of the quantization noise at a high frequency. With the halfband filter characteristics, as secondclass downsampling filter, respectively Kaiser window method and the best uniform approximation method to design halfband filter. Simulation filter designed by the two methods were selected after performance comparison of the best uniform approximation to design halfband filter. Further, taking into account the CIC within the passband frequency of the useful signal attenuation is relatively large, the design of a compensating filter for compensation processing, and the realization of the downsampling two times. The results show that systemlevel simulation of the overall filter passband cutoff frequency of 20 KHz, passband ripple of 0.005dB, stopband attenuation 93.4 dB downsampling rate of 64, to meet the design requirements.