looking for a sensor that activates when hit by a certain frequency burst
What you're looking for is an analog phase lock loop. This is the same type of circuit that will lock into the nearest FM radio station on your car radio. It works via a feedback loop that adjust an oscillator. 

You can also look at basic FM radio receiver circuits, and then manually tune the detector frequency to the frequency you're looking for. Instead of taking the output signal and listening to it, you can run it through a power detector (for example, look for a "Schottky diode Power detector") and then compare that (using a comparator) with a preset power level to know whether you detect a signal or not. A simple transistor radio is detailed here. Note that these circuits won't necessarily work on a wide range of RF frequencies, as FM radio only covers ~85MHz - 101Mhz or so.