The Use of a High-Speed 12-Bit A/D Converter for Digital Sampling of Communication Signal Carriers

Abstract

Abstract : The use of an A/D converter is considered for digital sampling of modulated waveforms obtained from an antenna. Demodulation is assumed to occur following the digital sampling process, either by using a high speed digital processor or by using a more conventional receiver following reconstruction of the analog waveform by a D/A converter. Since many communication carriers are not conditioned for jam-resistance by coding and spectrum-spreading techniques, a large instantaneous linear dynamic range is necessary to permit the detection and modulation of a small desired signal in the presence of a strong inband interfering signal. Consequently, our interest lies in A/D converters having a large number of quantization bits - a condition that is necessary for large linear dynamic range. The effects of A/D device parameters such as quantization resolution (number of bits), sampling rate, and aperture uncertainty are discussed. A 12-bit A/D converter having an aperture uncertainty of 25 psec and a maximum sampling rate of 5 MHz was used to experimentally determine the magnitude of the spurious-free dynamic range that could be obtained with present off-the-shelf technology. A larger number of quantization bits would have resulted in unacceptably slow speed and an unacceptably low maximum frequency limitation. A smaller number of bits would have yielded a smaller spurious-free dynamic range. Since we did not have a digital processing facility to analyze the A/D directly, a D/A converter, deglitcher (sample-and-hold), and a high quality spectrum analyzer were used to evaluate the A/D performance. Consequently, the results reported are characteristic of the composite A/D-D/A- deglitcher system rather than the A/D alone.