Frequency converters include waveguides configured for a local oscillator (LO) signal, an intermediate frequency (IF) signal, and an RF signal. A multimode IF waveguide can be used for selectively coupling of an IF signal and to reduce signal contributions produced by the LO signal. Typically, the multimode waveguide is situated to that the IF signal and the LO signal propagate in different waveguide modes, and a selected one of these signals can be selectively attenuated. In some examples, a periodically stepped waveguide is used to enhance propagation of a selected waveguide mode or a lossy conductor is used to attenuate a selected waveguide mode.