According to wikipedia, EMPs have a number of different waveforms. Radio transmitters are carefully designed with a near-perfect sine waveform so that their signals can later be filtered out by the receiver. A receiver's antenna is carefully designed with a specific length to receive signals within a particular frequency band. Standard AM/FM Radio frequencies are too close together to need to adjust the length of the antenna per station but in concept, only antennas of a certain length will pick up a radio signal of a particular frequency.
So then, could this idea work in reverse for a selective EMP weapon? Rather than designing an antenna to receive signals of a particular frequency, could an EMP be designed to only be received by devices containing circuits with a particular length? Or might there be some other method for designing an EMP's particular signal to affect only certain devices?
Hard science preferred but others are welcome in case there is no hard solution. Also open to variations on the EMP concept, like maybe this can be done with a continuous wave or etc.