FDMA/TDM multiple access/multiplexing system based on onboard FDMA-TDM conversion is believed to be very attractive for VSAT satellite communication networks, but it has two fatal drawbacks, i.e. its on board processing equipment is too complex and it lacks of flexibility in applications. This paper presents a new form of FDMA/CWTDM multiple access/multiplexing included in a Chinese patent, which can overcome the two drawbacks and suit broadband applications, where CWTDM means continuous wave time division multiplexing. In an onboard FDMA-CWTDM converter, how to implement the frequency-division demultiplexer is the most important issue. It is desired that its frequency band should be as wide as possible and the number of its channels be as great as possible. This paper emphasizes the research and design for such a demultiplexer. For the purpose, a polyphase array FFT demultiplexer module has been successfully developed based on FPGA design. Theoretical analysis and experimental results show that the demultiplexer module has very good performance and can find wide applications.