D.Yu. Denisenko - Post-graduate Student, Don State Technical University (Rostov-on-Don) E-mail: swallow5@mail.ru Yu.I. Ivanov - Ph. D. (Eng.), Associate Professor, Department of Automatic Control Systems, Academy of Engineering and technology of SFU (Taganrog) E-mail: таганрог@почта.рус N.N. Prokopenko - Dr. Sc. (Eng.), Professor, Head of Department «Information Systems and Radio Engineering», Head of Research Laboratory, Don State Technical University (Rostov-on-Don) E-mail: prokopenko@sssu.ru

On the base of computer simulation the qualitative and quantitative recommendations are given for choosing parameters of precision limiter of spectrum (of low-pass filter, antialiasing filter) for operation in the input circuit of analog-to-digital converter (ADC) of classical DSPS. We suggest making a choice of parameters of the low-pass filter (LPF) in such a way to obviate the need for its tuning. LPF is con-structed in two stages. At the first stage the requirements to the amplitude frequency characteristic (AFC) of LPF are formulated and the approximating task is solved. Then the order of transfer function of LPF and the character of AFC behavior in the bandpass ranges and attenuation bands are determined. At the second stage of construction we look for the circuit of LPF which allows realizing the found transfer function. The carried out analysis shows that the response rate of operational amplifiers and the tolerance for passive elements (resistors, ca-pacitors) cause the change of AFC of LPF mainly near its cutoff frequency. Due to this fact we suggest to choose the relative cutoff frequency of LPF 25−50% higher than the operating bandwidth of the signal processing system and to lead the high-sensitive AFC of LFP out of the operating band. However, it is possible only if the conversion frequency of ADC can be increased by 25−50% relating to cutoff frequency of LPF. As an example the results of simulation of the eighth order Chebyshev LPF are given at the spread of capacitor capacitance of ±1%, in-band ripple of 0.1 dB, stopband attenuation of 80 dB, cutoff frequency of LPF of 20 kHz, remanence ratio of amplitude frequency characteristic, equal to 2.3 when using starved operational amplifiers. The suggested choice of LPF parameters makes possible to refuse digital or analog setting of LPF parameters and to provide its oper-ation in the signal processing system with the specified characteristics.

 

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