The progress by which the signal being transmitted is reproduced from the radio-frequency currents present at the receiver is called detection, or sometimes demodulation. Where the intelligence is transmitted by varying the amplitude of the radiated wave, detection is accomplished by rectifying the radio-frequency currents.
The rectified current thus produced varies in accordance with the signal originally modulated on the wave radiated at the transmitter and so reproduces the desired signal.
Thus, when the modulated wave is rectified, the resulting current has an average value that
varies in accordance with the amplitude of the original signal.
In the transmission of code signals by radio, the rectified current reproduced the dots and dashes of the telegraph code and could be used to operate a telegraph sounder. When it is desired to receive the telegraph signals directly on a telephone receiver, it is necessary to break up the dots and dashes at an audible rate in order to give a note that can be heard, since otherwise the telephone receiver would give forth a succession of unintelligible clicks.
The detection of a frequency-modulated wave involves two steps.
First, the wave is transmitted through a circuit in which the relative output obtained from the circuit depends upon the frequency.
The circuit output is then an amplitude-modulated wave since, as the frequency of the constant-amplitude input wave varies, the output will vary correspondingly.
The resulting amplitude-modulated wave is then rectified.
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| Text 17. Reception of radio signals | | | Text 19. Difference between A.C. and D.S. |