A signal processing circuit is used for generating a signal output by processing a signal input, and includes a first mixer unit, a second mixer unit, and a frequency-selective combining block. The first mixer unit is arranged for receiving a first signal and a first oscillation signal, and generating a first mixing output by mixing the first oscillation signal and the first signal. The second mixer unit is arranged for receiving a second signal and a second oscillation signal, and generating a second mixing output by mixing the second oscillation signal and the second signal. Each of the first signal and the second signal is part of the signal input. The first and second oscillation signals have a same frequency but different phases. The frequency-selective combining block is arranged for frequency-selectively combining the first mixing output and the second mixing output to generate the signal output.