With the development of the modern communication system, broadband and high-frequency microwave radio frequency (RF) signals have been widely applied in the fields of radar, communication and signal processing. Based on the microwave photonic channelization, ultra-wideband RF signals were generated through dual optical frequency combs (OFCs) with different free spectrum ranges (FSRs). In the channelized synthesis system, multiple independent narrowband signal was input in each channel for up-conversion and detected by multi-heterodyne detection to reconstruct a wideband RF signal with continuous spectrum. In multi-heterodyne detection, interference suppression technique increased the highest frequency that the synthesized RF signal could reach. In the experiment, a wideband RF signal was synthesized with an instantaneous bandwidth of 4 GHz, covering a frequency range of 8.4-12.4 GHz. The experiment demonstrates an interference suppression ratio of 21 dB, indicating that the interference suppression technique increases the highest frequency of the output signal and effectively improves the spectrum utilization.