Locating non-linear components Elvira Lornet 24

An NLJD operation principle is based on illuminating a certain object under search with high power RF energy (either CW or pulsed) and on receiving the re-emitted object response at the multiples of the probing signal frequency (its second and third harmonics). The NLJD capability  of detecting hidden electronics comes from the non-linear properties of semiconductors. Any electronic device will contain some printed circuit boards (PCB's) with conductors (virtual antennas) to which various semiconductor elements (diodes, transistors, microchips) are connected. For a high frequency probing signal all these elements can be considered as non-linear reflectors. This high frequency probing signal will induce in these conductors an alternating emf being converted by  elements with a non-linear volt-ampere characteristic into RF signals on multiples (harmonics) of the probing frequency. These harmonics will be eventually re-emitted into space and detected by the NLJD's receivers tuned to these very frequencies. Detecting the 2-d and 3-rd harmonics of the probing signal by NLJD's receivers will mean that a hidden radioelectronic device is present in the illuminated area regardless of whether this device is powered on or switched off. It is conventionally assumed that the detected non-linear object is artificial in origin if the second harmonic's level exceeds that of the third one. If opposite is the case then the detected object is considered a natural non-linear junction of MOM-type (metal-oxide-metal). However, the NLJD application practice tells us that the mentioned criterion of the object origin identification may not always work well (e.g. a rusty metal element may exhibit a higher second harmonic level). In such a case additional identification methods may prove useful. This is where harmonics spectrum analysis will come really handy. In an ambiguous situation one may want to apply some physical impact on the object under search (e.g. knocking at it) while observing the second and third harmonics spectrum (or listening to the demodulated harmonics response in the headphones). The natural objects under a physical impact will show spectrum widening (regrowth) whereas the harmonics spectrum of artificial (electronic) object will remain largely unchanged. During demodulation the spectrum regrowth will manifest itself as a rustling noise in the headphones.