DOWN CONVERTER EMPLOYED IN A DIGITAL DEMODULATOR

PURPOSE: A down converter of a digital demodulator is provided to realize a decimation filter which improve processing speed of the down converter by applying polyphase filter theory to the decimation filter generating bottleneck. CONSTITUTION: A down converter(500) down-converts QAM signal transformed to digital samples in digital demodulator. The first multiplier(210) multiplies the digital samples by cosine coefficient to transform it to I samples of baseband. The second multiplier(310) multiplies the digital samples by sine coefficient(Fct) to transform it to Q samples of baseband. The first decimation filter(200) selects the I sample signal alternatively and adds the selected I samples to generates the selected baseband I signal. The second decimation filter(300) selects the Q sample signal alternatively and adds the selected Q samples to generates the selected baseband Q signal.