Nonlinear detectors

Non Linear Junction Detector - or an NLJD, is a device that illuminates a small region of space with high-frequency, RF energy. Any "non linear junction" in the vicinity—for example, and particularly, the p-n junction -- will receive this energy, and because of the non-linear nature of the junction, it will rectify it, re-emitting some of it on multiples of the illumination frequency (see harmonic). The detector has a sensitive receiver tuned for these harmonics, as well as appropriate processing and displays to make their presence known to the user of the device.

Active filters