The Principle of Detection

Author: J.B. Hoag

When a modulated carrier wave has been received, it is necessary to use a "detector" to separate, its audio component from its radio frequency component, if one is to hear the message sent into the microphone at the transmitter. In other words, the modulated wave must be demodulated. This may be accomplished by sending the wave through a non-linear device, i.e., through one whose current is not directly (linearly) proportional to the impressed voltage. For example, one may use certain crystals such as galena or iron pyrites which are much better conductors of current in one direction than in the other (roughly 10 to 1 better).

Fig. 17 A. A simple crystal receiver

In Fig. 17 A, the electromagnetic waves, cutting the antenna wires, induce an e.m.f. whose amount is strengthened by the tuned circuit. The e.m.f. across the condenser sends a current i through the crystal in greater amount in one direction than in the other, as indicated in Fig. 17 B.

Fig. 17 B. Principle of a crystal detector

The semi-direct-pulsating current stores energy in condenser C. This, in turn, feeds through the phones in an amount indicated by the dotted line in Fig. 17 B, whose wave form is like the audio-modulation component of the input wave, i.e., like the original audio sound waves at the transmitter.