Ron Bertrand VK2DQ http://www.radioelectronicschool.com AM TRANSMITTERS & RECEIVERS
Revision: our definition of amplitude modulation.
Amplitude modulation is when the modulating audio is combined with a radio frequency carrier is such a way that the total wave power is made to vary in accordance with the modulating audio.
The normal type of AM is double sideband or, to be more specific, double sideband full carrier. Recall that the carrier does not convey any intelligence to the receiver. The information is in the sidebands. Either sideband contains all of the information.
The advantage of double sideband full carrier is simplicity in reception. The disadvantages are that it is not very efficient power wise, and the bandwidth is greater than is necessary for other modes, particularly voice communications.
The carrier is used in the transmitter to heterodyne (mix) with the audio to convert the audio to radio frequencies (the sidebands), so that they can be radiated from an antenna.
The carrier is transmitted along with the sidebands. The advantage of this is that the carrier is available to the receiver to once again heterodyne with the sidebands, this time to convert the sidebands back to the original audio frequencies, which can then be heard.
If the carrier was not transmitted (SSB), then the receiver would have to generate its own carrier. Mixing or heterodyning in a transmitter is called modulating, whereas in a receiver it is called demodulating. The stage in a receiver which does the demodulating is called the demodulator though it is sometimes referred to by an older name, the detector.
The most basic AM (DSB full carrier) receiver is the crystal set. Let's have a look at how a crystal set operates. Figure 1 is the schematic diagram of a crystal set, part of which we discussed in tuned circuits.
The AM (double sideband full carrier) electromagnetic wave, and all the others in the air,