Conversion of analog signal into i and q digital signals with enhanced

Abstract

A system utilizing bandpass sampling for converting an analog input signal A.multidot.sin ((.omega..sub.o +.omega..sub.s)t+.phi.)) into (I) in-phase and a (Q) quadrature-phase digital signals for digital signal processing, in which image rejection is enhanced. .omega..sub.s is the baseband frequency, .omega..sub.o is the IF center frequency, and .phi. is the phase of the analog input signal. A sequence of digital signal samples obtained by bandpass sampling is processed to derive either the I digital signal or the Q digital signal as a first digital output signal from a first series of alternate samples and to derive the other of the I and Q digital signals as a second digital output signal by computing the average of the absolute values of alternate samples that immediately precede and follow the samples of the first series. In accordance with this processing scheme, there is no phase error, but instead an amplitude error in each I/Q pair equal to the value of cos .omega..sub.s .tau.; and the image rejection becomes: ##EQU1## For an UHF receiver with 25 KHz wide channels, the maximum baseband frequency is 12.5 KHz. When sampled at f=4 MHz, the image rejection is 80 dB.