DETECTION

A diode can recover the audio signal from radiofrequency (rf) ac. This is called demodulation or detection. If diode detection is to be effective, the diode should have low junction capacitance. Then it works as a rectifier at rf, passing current in one direction but not in the other.
Some rf diodes are microscopic versions of the old cat whisker, in which a fine wire was placed in contact with a crystal of a mineral called galena. Components of this type are known as point-contact diodes and are designed to minimize the junction capacitance. In this way, as the frequency becomes higher and higher (up to a certain maximum), the diodes keep acting like rectifiers rather than starting to behave like capacitors. This makes point-contact diodes good for use at rf.