Detection at Semiconductor Junctions

Every semiconductor junction, whether part of a diode,transistor, or integrated circuit, is quite nonlinear, especially in the voltage region where it is beginningto conduct. In analog circuits, we prevent this non-linearity from causing distortion by properly biasing the circuitry, by using lots of negative feedback, and by preventing the signal from being large enough to cross into the cutoff region.

Thanks to this non-linearity, every semiconductor junction functions as a square law detector, detecting any RF signal it sees. A good designer prevents detection by shielding the equipment and its wiring, by filtering input and output wiring, and even by bypassing the junction by a capacitor.

Since virtually all detection that causes RFI follows square law, the strength of the signal detected by audio equipment, telephones, and other equipment will increase (or decrease) as the square of any increase (or decrease) in RF level at the detector. In other words, the strength of the detected RF changes by twice the number of dB that the RF signal changes. This means that if we manage to reduce the interfering RF signal by 6 dB, the detected audio will drop by 12 dB. This is a very useful
thing – it means that we may not need "an elephant gun" to solve many interference problems.