Non-Linear Junction Detection is a well-known technique for detecting electronics that utilize semiconductor (solid-state) junctions. The current state of the art for finding hidden electronics — such as electronic eavesdropping devices — using this technology has a maximum range of about 2 m, and more typically between 6 and 12”.
Electronic devices typically contain multiple nonlinear junctions linked by wires or traces to other components. Therefore, energy may couple into and out of the device through multiple paths; moreover, the path(s) into a device may be different than the path(s) out of the device. It is to be noted that powering a device may alter its coupling characteristics (that is, biasing a diode of interest will place a signal farther up the IV curve).
Spread spectrum techniques are commonly used in communications, as they provide high sensitivity for low-power requirements. Examples include cellphones and GPS (the latter uses a 50-W transmitter 20,000 km away). Pseudo-random encoding at the transmitter and cross-correlation at the receiver are used to detect and locate extremely weak signals, even in a noisy RF environment — this is how dozens of cellphones can work in close proximity without interfering with one another. Multiple techniques exist, including phase shift keying (PSK), frequency shift keying (FSK), amplitude shift keying (ASK), and the like.
An apparatus for remotely locating solid-state electronics has been developed. It includes:
RF generator for generating a chosen frequency RF signal
Signal splitter for dividing the RF signal into a transmitted portion and a reference portion
Modulator for encoding a chosen code onto the transmitted portion of the RF signal
Controller for generating the chosen code and for directing the code to the modulator
Power amplifier for amplifying the encoded RF signal
Antenna for transmitting the amplified, encoded RF signal
Antenna for receiving a similarly encoded second harmonic frequency of the encoded RF signal
Demodulator for receiving the second harmonic frequencies and the chosen code delayed by a selected time interval from the controller, and for removing the modulation from the second harmonic frequency
Frequency doubler for doubling the frequency of the reference portion of the RF signal
Mixer for comparing the doubled frequency of the reference portion of the RF signal with the demodulated second harmonic frequency, and for generating a DC signal if the doubled frequency of the reference portion of the RF signal is correlated with the demodulated second harmonic frequency
Computer for receiving the DC signal, for directing the controller, and for calculating distance between located solid-state electronics and the transmitting antenna
The system displays sufficient performance enhancement that the apparatus will perform at distances of 100 m on the ground. In addition, the manner in which detection is performed provides range information, thereby greatly increasing the ability to discriminate targets, as well as precisely locating targets.