Direct RF Sampling Employing Time-Skewed Analog to Digital Converters with Complex Finite Impulse Response Filters

Abstract

This paper presents an approach to directly sample and baseband an RF signal without the use of mixers. The technique utilizes an array of time-skewed analog to digital converters (ADC) combined with complex finite impulse response (FIR) filters to provide digital I and Q samples of the complex envelop of the RF signal. Since the process is complex, the analog to digital conversion rate need only be commensurate with the signal bandwidth, and not the carrier frequency. This device is referred to as a complex analog to digital converter, or simply, CADC. The CADC is based on the use of FIR filters with complex coefficients. These can be used to filter and demodulate a sampled signal of arbitrary bandwidth to baseband-without the use of a demodulator or mixer. The CADC uses aliasing to effectively demodulate the signal of interest to baseband, which obviates the complex demodulators often used in sampled data systems. For high frequency applications, such as radar, this can eliminate the need for analog mixers often used to mix frequencies to an intermediate frequency (IF) prior to analog to digital conversion. Since the conversion rate need only be commensurate with the signal bandwidth, slower ADCs can be used with respect to other direct RF sampling methods. These slower ADCs tend to have more effective number of bits (ENOB) than those which operate at higher rates, resulting in the wider dynamic range often desired in radar applications. The CADC architecture is more immune to ADC matching errors such as amplitude, phase, and DC offset, which are often encountered with the more traditional time-interleaved ADC arrays. The CADC also reduces the impact of jitter and quantization noise because of its filtering characteristics. The filtering thus acts to increase the ENOB over that of each ADC.