Digital quadrature amplitude modulator

Abstract From the Canadian patent

In a quadrature amplitude modulator two binary, syn-, chronous data signals representing four possible phase states of a carrier signal are sampled and respectively fed into two shift registers comprising part of a finite impulse response lowpass filter. The samples in each corresponding pair of stages in the shift registers are multiplexed at twice the carrier frequency. Each multiplexer output is then added modulo-two to a binary signal at the carrier frequency using exclusise-OR gates. The gate outputs are weighted in a desired fashion and summed and converted to an analog signal which exhibits a preferred spectral shape. This approach enables the quadrature amplitude modulation function …