A diplexer includes a first filter circuit, and a second filter circuit. The first filter circuit is coupled to a first port for providing a first signal path for a first radio frequency (RF) signal. The second filter circuit is coupled to the first port for providing a second signal path for a second RF signal. The first filter circuit includes a first tunable resonant circuit for tuning a first transmission zero corresponding to a first frequency multiplication of the first RF signal. The second filter circuit includes a second tunable resonant circuit for tuning a first transmission zero corresponding to a first frequency multiplication of the second RF signal. The first frequency multiplication of the first RF signal corresponding to the first filter circuit is a fourth harmonic of the first RF signal.