Detection

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Schematic diagram of a crystal-set radio receiver.
One of the earliest diodes, existing even before vacuum tubes, was actually a primitive semiconductor device. Known as a cat whisker, it consisted of a fine piece of wire in contact with a small piece of the mineral galena. This strange-looking contraption had the ability to act as a rectifier for extremely weak RF currents. When the cat whisker was connected in a circuit such as the one shown in above figure, the result was a device capable of picking up amplitude-modulated (AM) radio signals and producing audio output that could be heard in the headset.
 
The galena, sometimes called a “crystal,” gave rise to the nickname crystal set for this primitive radio receiver. You can still build a crystal set today, using a simple RF diode, a coil, a tuning capacitor, a headset, and a long-wire antenna. Notice that there’s no battery! The audio is provided by the received signal alone.
 
The diode in above figure acts to recover the audio from the radio signal. This process is called detection; the circuit is called a detector or demodulator. If the detector is to be effective, the diode must be of the proper type. It must have low junction capacitance, so that it can work as a rectifier (and not as a capacitor) at radio frequencies. Some modern RF diodes are microscopic versions of the old cat whisker, enclosed in a glass case with axial leads.