In radar system, the signal system of signal source and the mode of signal generation play an important role in radar performance. According to the specific request of through-wall imaging radar (TWIR) systems, this paper chooses the stepped frequency continuous waveform(SFCW) radar, and uses the technology of phase locked logic(PLL) frequency synthesis to synthetize the stepped frequency signal. Stepped frequency is a common waveform used in throughwall imaging radar. Traditional stepped frequency throughwall imaging radar, that uses PLL frequency synthesis technology, has a long frequency hopping time and low frame rate. This paper proposed a novel scanning mode, which could evidently enhance the frame rate of TWIR. With 2MHz step size and 2.5GHz frequency range, the real imaging frame rate could be 14Hz. To some extent, it could meet the requirements of real time imaging. The proposed approach is proved in experiment.Index Terms -Stepped Frequency, PLL frequency synthesis, scanning mode, channel switching, frame rate, FPGA.