A location is identified of at least one PIM (passive intermodulation) source in a frequency selective device by applying an excitation waveform to the frequency selective device and measuring a PIM response signature of the frequency selective device. The PIM response signature is a characteristic of PIM produced in response to the excitation waveform. The measured PIM response signature is compared with each of a plurality of example PIM response signatures, each of the plurality of example PIM response signatures corresponding to a characteristic of PIM expected for a respective location of a PIM source in the frequency selective device. The location of the at least one PIM source within the frequency selective device is determined on the basis of the comparison.