This paper presents a noise reduction technique for Terahertz imaging based on modulating the THz source in order to transpose the signal at the output of the rectifier at a higher frequency where there is no 1/f noise and then filtering with a high Q bandpass filter centred at the modulation frequency. The filtering circuit, based on a 16-path passive switched capacitor (SC) bandpass filter, is modeled analytically. The presented technique is implemented in a test chip with a 130 nm process. Measurement results validate the analytical results and show that a Q factor of 100 is achieved leading to a noise reduction of more than 20dB.