Abstract: An apparatus for determining unknown radio frequencies is described. The apparatus modulates a first optical signal from tunable optical source with RF signal that includes unknown center frequency value to generate modulated optical signal. The apparatus combines modulated optical signal with second optical signal to generate combined modulated optical signal. The apparatus receives output electrical signal from photodetector in response to receiving combined modulated optical signal at photodetector, splits output electrical signal into first output electrical signal and second output electrical signal, and filter first output electrical signal by applying first optical bandpass filter centered to generate first filtered RF signal. The apparatus tunes tunable optical source to first beat frequency value to determine unknown frequency value of RF signal.