Radio transmissions are spectrally contained to reduceadjacent channel interference by a method and apparatus forI, Q filtering of signals before quadrature modulation indigital cellular radio transmission systems such ascommunications systems following the GSM TDMA, IS-54 or IS-95standards. The filtering is accomplished by multiple stageregisters connected at their complementary outputs toidentical resistor networks. The outputs of the resistornetworks are summed to represent a smoothly transitioninganalog representation of the digital voice or datatransmission. The analog representation may be additionallylow pass filtered to remove higher frequency componentsbefore quadrature modulation. Because rapid transmissions inthe transmittal signal are avoided, adjacent channelinterference is reduced.
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