A digital modulator and digital demodulator with quadrature amplitude modulation (QAM) schemes, which are designed to modulate or demodulate RZ-coded baseband signals. The digital modulator comprises first to fourth roll-off filters and a first and second inverters connected to the second and fourth roll-off filters. It also comprises a parallel-to-serial converter to successively selects the outputs of the first roll-off filter, third roll-off filter, first inverter, and second inverter. A D/A converter converts the selected digital signal stream into an analog signal. The roll-off filters and inverters operate at a predetermined clock frequency, while the parallel-to-serial converter and the D/A converter work at a frequency four times the predetermined clock frequency. The digital demodulator reverses the above modulation process to reproduce the baseband signals.
There is provided a quadrature modulation transmitter which is capable of solving several problems of the conventional transmitter while performing the same function as the heterodyne transmitter or the digital IF transmitter, in which a circuit structure is simplified and a power consumption is reduced compared with the conventional transmitter. The quadrature modulation transmitter includes: a digital processing block for receiving an I-channel data, a Q-channel data and a clock signal, modulating the I-channel data or an inverted I-channel data into a first analog signal by means of an I-channel DAC according to a switching of an I-clock signal identical to the clock signal, and modulating the Q-channel data and an inverted Q-channel data into a second analog signal by means of a Q-channel DAC according to a switching of a Q-clock signal, the Q-clock signal being an inverted clock signal; and an analog processing block for receiving the first and second analog signals from the digital processing block, adding the first and second analog signals, converting the added signal into an RF domain signal through a mixing operation, and amplifying and transmitting the RF domain signal.
