A low-noise block downconverter includes a first down-converting circuit for down-converting and filtering a first polarization signal for outputting a first intermediate-frequency signal, a second down-converting circuit for down-converting and filtering a second polarization signal for outputting a second intermediate-frequency signal, wherein a frequency band of the first intermediate-frequency signal is the same as that of the second intermediate-frequency signal, an oscillator for generating an oscillating signal, outputted to the first down-converting circuit and the second down-converting circuit, a first optical transmitter coupled to the first down-converting circuit for converting the first intermediate-frequency signal into a first optical signal, and a second optical transmitter coupled to the second down-converting circuit for converting the second intermediate-frequency signal into a second optical signal.