Direct digital downconverter and method for converting an analog signal to a digital signal

Abstract

A control scheme for operating an oscillator/counter A/D converter so that it simultaneously provides frequency downconversion, band pass filtering and analog-to-digital conversion of an analog signal, where the analog signal includes a carrier wave modulated with information by any known modulation technique. The converter uses a superconducting, Josephson single flux quantum circuit operating as a voltage controlled oscillator. The voltage controlled oscillator receives the analog signal to be converted, and generates a series of sharp, high frequency pulses based on the characteristics of the carrier signal. The series of pulses are applied to a gate circuit that, depending on a gate control signal, either blocks the pulses or passes the pulses to either an increment or a decrement port of a digital counter. When the pulses are passed by the gate circuit, the counter circuit accumulates the pulses during a sampling period. The sampling period covers a range of gate control pulses so that the accumulation of pulses defines consecutive on/off periods of the gate control signal. The scheme provides for frequency conversion and eliminating or minimizing the amount of DC response present in the digital output signal.