A frequency demultiplexer comprising an input part (106) with an input port (101), a low pass filter (125) and a band-pass filter (108) with output ports (120, 145). The input part (106), the low-pass filter (125) and the band-pass filter (108) comprise open waveguide sections, and the band-pass filter (108) comprises gap-coupled resonators (130, 135, 140). The input part (106) and the low-pass filter (125) connect to the same resonator (130), the connection (121) of the low-pass filter (125) being at a first maximum distance (L1) from a center point (N) of the resonator and the connection (116) of the output port (101) being at a second maximum distance (L2) from said center point (N) of the resonator. The center point (N) corresponds to a wave node of a wavelength λ, where λ=2d/M, M is a positive integer value and d is the shortest end-to-end distance along the resonator.