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

The use of an AD 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 DA 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 AD converters having a large number of quantization bits - a condition that is necessary for large linear dynamic range. The effects of AD device parameters such as quantization resolution number of bits, sampling rate, and aperture uncertainty are discussed. A 12-bit AD 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 AD directly, a DA converter, deglitcher sample-and-hold, and a high quality spectrum analyzer were used to evaluate the AD performance. Consequently, the results reported are characteristic of the composite AD-DA- deglitcher system rather than the AD alone.