3.8: SDR Quadrature Demodulator

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The SDR circuit used to demodulate an I/Q modulated RF signal is very similar to the standard quadrature demodulator. Complete demodulation and separation of the I channel and the Q channel is obtained when the LO of the demodulator coincides with the RF carrier frequency, see Figure 3.10.24(a). If the LO is not equal to the carrier frequency the RF modulated signal will be down-converted to an intermediate frequency signal which corresponds to a band around the difference frequency of the carrier and the LO, and a band around the sum frequency of the carrier and the LO, see Figure 3.10.24(a). Then lowpass filtering would be used to eliminate the higher frequency band. The signal centered at the intermediate frequency can then be I/Q demodulated by another quadrature demodulator.