an input port to receive an input microwave or RF signal;

a laser to produce a continuous-wave laser beam;

a first optical path to receive a first portion of the laser beam;

a second optical path to receive a second portion of the laser beam, the second optical path comprising:

an optical modulator to modulate the second portion in response to the input signal to produce a modulated optical beam that carries the input signal, and

a tunable optical filter to filter the modulated optical beam to select at least one spectral component in the input signal while rejecting other spectral components and to output a filtered modulated optical beam that carries the at least one selected spectral component, the tunable optical filter comprising two optical resonators that are optically coupled to produce a filter function of at least a second order;

a tuning control unit to tune at least one of the two optical resonators to chance a frequency of the at least one selected spectral component; and

an optical detector to combine the first portion from the first optical path and the filtered modulated optical beam from the second optical path and to produce a filtered output signal comprising the at least one selected spectral component,

wherein the two optical resonators are whispering gallery mode resonators and are tunable via an electro-optic effect, the tunable optical filter further comprising a third electro-optic whispering gallery mode resonator optically coupled to one of the two tunable optical resonators and tuned by the tuning control unit to effectuate a third order filter function in the tunable optical filter.